Considered one of the finest climbers in Britain in the decades after World War II, Dr. Ward later became a renowned authority on high-altitude medicine. For many years a lecturer in clinical surgery at London Hospital Medical College, he practiced in various hospitals in the East End of London.

Dr. Ward was the author of several books on mountaineering and its physiological effects, among them ''Everest: A Thousand Years of Exploration'' (Ernest Press, 2003) and a memoir, ''In This Short Span'' (Gollancz, 1972). With James S. Milledge and John B. West, he wrote a seminal textbook, ''High Altitude Medicine and Physiology'' (University of Pennsylvania, 1989).

The highest mountain in the world at 29,035 feet, Mount Everest straddles the border between Tibet and Nepal. During the first half of the 20th century, climbers could approach it only from the north -- the Tibetan side. But after the Chinese invasion of 1950, Tibet was placed off limits to foreigners. Around this time, Nepal, which had long been closed to outsiders, began to ease its own restrictions. Cartographers and ethnographers rushed to get in, and so did mountaineers.

''Every mountain has at least one way up it,'' Dr. Ward told The Independent of London in 1995. ''We reckoned there must be a route up Everest from Nepal.''

Michael Phelps Ward was born in London on March 26, 1925. As a boy, he became enthralled with climbing after reading ''Camp Six'' (Hodder & Stoughton, 1937), by F.S. Smythe, an account of the unsuccessful British attempt on Everest in 1933. Dr. Ward was educated at Cambridge University and London Hospital Medical College.

In the early 1950's, Dr. Ward, then a young doctor with the Royal Army Medical Corps, began sifting through the chaotic archives of the Royal Geographical Society. He came upon a cache of aerial photographs of Everest's south face, many taken covertly by the British in the 1940's. He also unearthed a map, known as the Milne-Hinks map, which had been compiled before the war and included what was then known of the mountain's south side.

A keen reader of mountains, Dr. Ward believed the images showed a clear way to the top. With William Murray, a noted Scottish mountaineer, he approached the Joint Himalayan Committee of the Alpine Club and the Royal Geographical Society. The two men asked the committee to sponsor a reconnaissance expedition up Everest's south face in 1951.

Dr. Ward was a member of that expedition, which ascended the mountain from the south, picking its way through the treacherous Khmubu ice fall. Also in the party was a young beekeeper and mountaineer from New Zealand named Edmund Hillary. Though the expedition turned back well before the summit, there was no doubt that Dr. Ward had found a viable route.

Studying earlier assaults on Everest, Dr. Ward concluded that they had failed because the medical consequences of altitude were poorly understood. Of particular concern on Everest were the last thousand feet, which climbers -- even those carrying oxygen -- typically ascended in a sickly stupor.

Dr. Ward assisted Griffith Pugh, a noted British physiologist, in researching medical aspects of diet, oxygenation and hydration at high altitude. Their recommendations included increasing the supply of oxygen for each climber.

On March 10, 1953, the historic expedition, led by Col. John Hunt, set out from Katmandu, painstakingly ascending Everest by the route Dr. Ward identified. On May 29, Edmund Hillary and Tenzing Norgay stepped onto the top of the world.

Dr. Ward never attained the summit. As the expedition's official doctor, he had to remain some 1,500 feet below, at Camp Seven, on the mountain's Lhotse Face. Besides treating the occasional sore throat, he later said, ''basically I had nothing to do.'' He caught up on Jane Austen and the Brontės.

In the early 1960s, Dr. Ward took part in the Silver Hut research project in the Everest region. There, he spent months living at 19,000 feet and later lived alone at 25,000 feet, where he pedaled a stationary bicycle to study the effects of altitude. In 1980, he helped lead a reconnaissance expedition to Mount Kongur, a remote, unclimbed mountain in China more than 25,000 feet high. Besides his wife, the former Felicity Jane Ewbank, whom he married in 1957, Dr. Ward is survived by their son, Mark, of Los Angeles, and one grandchild. In 1983, he was made a Commander of the British Empire.