WASHINGTON — Former President Clinton yesterday underwent a nearly four-hour operation to remove what was described as a "thick, rubbery, plaque-like ... peel" from the surface of his left lung. His surgeons at New York-Presbyterian Hospital in New York pronounced the procedure a success.

"We expect full function recovery of that lung," said Dr. Joshua Sonett, the chest surgeon who performed the operation.

Surgeons had hoped to do their work by inserting instruments through small incisions between two ribs while looking through a fiberoptic videoscope. However, their initial inspection showed that the scar tissue was too extensive to be removed that way. Consequently, they did the operation "open" — through a larger incision, with lung surface viewed directly.

The "pleural peel" — the name for the thickened part of the pleura, or smooth membrane covering the lungs — was nearly one-quarter-inch thick in some places, according to the surgical team. Its size and toughness was one of the reasons the procedure took longer than the one to three hours originally predicted. There were no complications, however.

Clinton's condition is a rare complication of coronary artery bypass surgery, which he underwent Sept. 6. Why it occurs is unknown.

Fluid accumulated around the base of his left lung, compressing it and reducing his breathing capacity by about 25 percent. It caused him slight discomfort while walking up hills, which he is doing as part of a post-bypass rehabilitation program. It also made him somewhat winded.

The condition was not life-threatening. His doctors diagnosed it before he went to South Asia last month with former President George H.W. Bush to meet victims of the Dec. 26 tsunami and raise money for relief efforts.

Clinton, 58, is expected to be in the hospital up to 10 days.