Arnall Patz, 89, an emeritus professor of ophthalmology at Johns Hopkins University who discovered in the 1950s what was then the most common cause of blindness in premature infants, died March 11 of heart disease at his home in Pikesville, Md.

Dr. Patz was a young physician-in-training at Washington's Gallinger Municipal Hospital when he and a colleague, Leroy Hoeck, teamed up to tackle one of the most vexing medical mysteries of the post-World War II era. An unusually high number of premature babies were becoming blind after their long hospital stays, and no one knew why.

Dr. Patch and Hoeck thought supplemental oxygen, which doctors administered routinely to help premature infants breathe, was to blame. Their hunch was correct, and almost immediately, doctors stopped automatically giving oxygen to premature infants, ending the epidemic of blindness because of retrolental fibroplasia, now known as retinopathy of prematurity.

To prove their theory, the pair of doctors conducted what is widely believed to be the first randomized controlled trial in ophthalmology. In the early 1950s, they divided 120 premature babies at Gallinger into two groups. In the first group, which received concentrated oxygen constantly, 12 infants went blind. In the second group, babies received oxygen only if they were in respiratory distress, and only one became blind.

Elevated oxygen levels, it turned out, destroyed the arteries of the eye. That in turn caused abnormally wild growth of blood vessels, irreversibly damaging the retina.

In showing that a therapy as beneficial as oxygen could have unintended consequences, the two unknown researchers shocked the medical community and helped prove the importance of testing all treatments with randomized trials.

"Doctors have to approach their patients, and what they think they know, with a certain amount of humility," Steven Goodman, a physician at Johns Hopkins's Bloomberg School of Public Health, told The Washington Post in 2005. "This is one of the trials that taught us humility."

The results of a subsequent larger trial, led by biochemist Everett Kinsey and involving patients at 18 hospitals, substantiated the earlier findings at Gallinger. Although the new understanding came too late for thousands of people who were made blind by oxygen, including the singer Stevie Wonder, it undoubtedly saved many more from a similar fate.

Dr. Patz went on to collaborate with colleagues at the Johns Hopkins Applied Physics Laboratory to develop one of the first argon lasers used to treat diabetes-related eye disease and other retinal problems. He and Kinsey received the Albert Lasker Clinical Medical Research Award, dubbed the American Nobel Prize, for his groundbreaking research. Helen Keller presented them with the honor in 1956.

In 2004, President George W. Bush presented Dr. Patz with the Presidential Medal of Freedom, the nation's highest civilian award, calling him "the man who has given to uncounted men, women and children the gift of sight."

Arnall Patz was born June 14, 1920, in rural Elberton, Ga., the youngest of seven siblings and the grandson of Jewish immigrants from Lithuania. He received bachelor's and medical degrees from Emory University in Atlanta, graduating from medical school in 1945.

He joined the Army and served at Walter Reed Army Medical Center, where he decided he wanted to be an ophthalmologist. When he was discharged from the military in 1948, he began a residency in ophthalmology at Gallinger, which later was renamed D.C. General Hospital.

When he and Hoeck applied to the nascent National Institutes of Health for a grant to pay for their pioneering work on premature-baby blindness, they were turned down on ethical grounds. "These guys are going to kill a lot of babies by anoxia [inadequate oxygen] to test a wild idea," one grant reviewer wrote, according to the Baltimore Jewish Times.

They funded their early tests with money borrowed from Dr. Patz's brother, Louis, and later received a small grant after promising to turn on the oxygen at the first sign of troubled breathing. Louis Patz and his wife were killed in a 1962 airplane crash. Their three children were raised by Dr. Patz and his wife.

Dr. Patz became a part-time member of the Johns Hopkins faculty in 1955, maintaining a private ophthalmology practice for 15 years until becoming a full-time research professor in 1970. In 1979, he became the director of the Wilmer Eye Institute at Hopkins, a position he held for 10 years. He also operated a ham radio from his home on behalf of the Maryland Eye Bank.

Dr. Patz was a talented fly fisherman who was so successful in his pursuit of trout that he was eventually banned from participating in the American Ophthalmology Society's annual fly-fishing contest. He spent many summers at the cabin he built with his family in Maine's North Woods, 15 miles down a dirt road on a lake thick with trout.

An endlessly curious person who had completed his bachelor's degree on an accelerated wartime schedule, he took the opportunity later in life to study Beethoven's music and world religions. At age 78, he received a master's in liberal arts from Johns Hopkins.

Dr. Patz died one day before his 60th anniversary. His wife, the former Ellen Levy, lives in Pikesville. Other survivors include four children, William Patz of Seattle, Susan Patz of Baltimore, David Patz of Grand Junction, Colo., and Jonathan Patz of Madison, Wis.; three nieces and nephews whom Dr. Patz and his wife raised, H. Samuel Patz of Boston, Harry Patz of Acme, Wash., and Sarah Anne Patz of St. Louis; and eight grandchildren.