WASHINGTON — When Sandi Stokes waits for lunch at the sandwich shop near her office in downtown Washington, she notices the counter worker often assumes the white person next to her was there first.

Brenda Person frequently finds that when she goes shopping near her home in Silver Spring, Md., clerks seem to ignore her and instead help a white customer.

Peggy Geigher, a District of Columbia resident, says restaurant hostesses often seem to seat her near the bathroom, even when better tables are available.

Many African Americans tell stories like these — seemingly minor examples of subtle discrimination they experience routinely.

"It happens all the time," said Person, 56, a mother of two daughters. "It's part of day-to-day experiences, unfortunately. But you are never prepared for it — it makes you feel like you're out of rhythm with the rest of the world, and like there's no justice."

Some medical researchers have begun to suspect that such incidents take a physical toll and may play a role in why black people tend to have poorer health than white people. Chronic, low-level stress from such incidents may increase the risk for a host of ills, including heart disease and cancer, according to the theory.

The hypothesis remains far from proven and is highly controversial. Skeptics say it is difficult to rule out other factors, such as diet, lifestyle, personal perceptions and cultural differences. But support for the theory has been accumulating slowly, including a new study released yesterday linking such experiences to the early stages of heart disease. Some researchers say it is among the strongest pieces of evidence so far.

"Exposure to this kind of discrimination has real health implications for African Americans," said Tene Lewis, a health psychologist at Rush University Medical Center in Chicago who presented the findings at an American Heart Association meeting in Washington. "We need to figure out how it's happening and why it's happening and how we can prevent it from happening."

Health experts have known for decades that blacks and other racial minorities are at far greater risk than whites for many health problems, tend to suffer more complications when they are sick and are less likely to recover. Most of the differences can be explained because blacks tend to be poorer and have less access to high-quality health care, experts say. Even when socioeconomic factors such as income and education are accounted for, though, blacks still tend to fare worse, leading some to suspect that stress from insidious, persistent discrimination may play a role.

Studies have linked discrimination to higher rates of depression, and a small number have found associations with risk factors for physical disease, such as high blood pressure. The new study is the largest, most detailed to date to examine the relationship between discrimination and the early stages of a physical ailment.

Lewis and her colleagues studied 181 black women ages 45 to 58 in Chicago and Pittsburgh who are participating in a large, ongoing project, called the Study of Women's Health Across the Nation (SWAN), that is examining a host of health issues among middle-aged women.

As part of the SWAN project, every year between 1996 and 2001 the women answered a questionnaire designed to measure encounters with subtle racial discrimination.

"We're not talking overt incidents. It's not racism in the form of being chased down the street because you have brown skin or being called a name," Lewis said. "We're talking about subtleties — everyday insults that build up over time."

The women's scores over four years were averaged on a four-point scale, and in 2001 the participants underwent an examination known as a CT scan to measure coronary artery calcification — buildup of calcium inside arteries that supply blood to the heart. It is considered an early stage of heart disease — the nation's leading cause of death.

The more discrimination reported, the more likely they were to have calcification, researchers found. After accounting for age, geographic location and education, researchers found that for every unit of increase in perceived discrimination, the odds of having calcification nearly tripled. The chances of having calcification remained 2 ½ times higher even after researchers took into consideration such factors as high blood pressure, cholesterol, smoking, age and body weight.

"It's a strong association," Lewis said before her presentation.

Although the study involved only women, the findings likely would hold true for men, and possibly for other ethnic groups as well, Lewis said. "We're seeing a real risk factor here."

Other research has suggested that chronic stress may increase the risk of heart disease by raising levels of stress hormones, such as cortisol, as well as boosting levels of inflammation in the body.

"We believe it's the accumulated burden of this subtle racial discrimination that's having this effect," Lewis said. "It's not just having this experience once or twice but having it over and over again throughout a person's life."

Other researchers agreed, noting that stress can also suppress the immune system.

"I think the findings clearly indicate that racism matters a lot," said David Williams, an expert on racial disparities in health at the University of Michigan in Ann Arbor. "A lot of people dismiss reports of discrimination as just something that's in people's minds. What these data suggest is these minor incivilities and minor incidents of discrimination are actually consequential for physiologic function and adversely affect health."

Norman Anderson, chief executive officer of the American Psychological Association who has done research in this area, called the study "groundbreaking."

Others were skeptical, saying the study failed to differentiate whether the women actually had experienced discrimination or just thought they had.

"It's almost meaningless," said Sally Satel, a resident scholar at the American Enterprise Institute who is a vocal critic of such research. "If someone is rude to you, you don't know if it's discrimination or your perception. People who are already high-strung and hypervigilant may bring that kind of interpretation to the situation and are probably the kind of people who would be at high risk for heart problems."