Koop personally opposed homosexuality and believed sex should be saved for marriage. But he insisted that Americans, especially young people, must not die because they were deprived of explicit information about how HIV was transmitted.
Koop further angered conservatives by refusing to issue a report requested by the Reagan White House, saying he could not find enough scientific evidence to determine whether abortion has harmful psychological effects on women.
Koop maintained his personal opposition to abortion, however. After he left office, he told medical students it violated their Hippocratic oath. In 2009, he wrote to Senate Majority Leader Harry Reid, urging that health care legislation include a provision to ensure doctors and medical students would not be forced to perform abortions. The letter briefly set off a security scare because it was hand delivered.
Koop served as chairman of the National Safe Kids Campaign and as an adviser to President Bill Clinton's health care reform plan.
At a congressional hearing in 2007, Koop spoke about political pressure on the surgeon general post. He said Reagan was pressed to fire him every day, but Reagan would not interfere.
Koop, worried that medicine had lost old-fashioned caring and personal relationships between doctors and patients, opened his institute at Dartmouth to teach medical students basic values and ethics. He also was a part-owner of a short-lived venture, drkoop.com, to provide consumer health care information via the Internet.
Koop was born in the New York City borough of Brooklyn, the only son of a Manhattan banker and the nephew of a doctor. He said by age 5 he knew he wanted to be a surgeon and at age 13 he practiced his skills on neighborhood cats.
He attended Dartmouth, where he received the nickname Chick, short for "chicken Koop." It stuck for life.
Koop received his medical degree at Cornell Medical College, choosing pediatric surgery because so few surgeons practiced it.
In 1938, he married Elizabeth Flanagan, the daughter of a Connecticut doctor. They had four children, one of whom died in a mountain climbing accident when he was 20.
Koop was appointed surgeon-in-chief at Children's Hospital in Philadelphia and served as a professor at the University of Pennsylvania School of Medicine.
He pioneered surgery on newborns and successfully separated three sets of conjoined twins. He won national acclaim by reconstructing the chest of a baby born with the heart outside the body.
Although raised as a Baptist, he was drawn to a Presbyterian church near the hospital, where he developed an abiding faith. He began praying at the bedside of his young patients — ignoring the snickers of some of his colleagues.
Koop's wife died in 2007, and he married Cora Hogue in 2010.
He was by far the best-known surgeon general and for decades afterward was still a recognized personality.
"I was walking down the street with him one time" about five years ago, recalled Dr. George Wohlreich, director of the College of Physicians of Philadelphia, a medical society with which Koop had longstanding ties. "People were yelling out, 'There goes Dr. Koop!' You'd have thought he was a rock star."
Ring reported from Montpelier, Vt. Cass reported from Washington. AP Medical Writers Lauran Neergaard in Washington and Mike Stobbe in New York contributed to this report.