An inquisitive medical student was examining women at a Hong Kong clinic.

He began to notice a peculir thing. A number of these women had a plainly larger right breast than left.

Further, all of the women with the lopsided breasts were Tanka, boat people, wives in fishing families that had lived on the water for generations. It turned out that these women - for convenience, they said - breast-fed their many babies only from their right breasts.

The observant young man, Roy Ing. is now a doctor and since 1974 an investigator as the U.S. Public Health Service Hospital in Baltimore.

His observation has led him to find that these women, alone on the globe, tend to develop cancer in their late years in their smaller, unused breast. And the discovery has reopened one of medicine's long-standing questions: Does breast-feeding protect against breast cancer.

The finding also helps suggest that breast-feeding may clear carcinogenic or cancer-causing chemicals from the breast, according to a report by Ing and two co-workers in the mid-June issue of Lancet, a respected British medical journal.

It also prompts Ing to say. "If my wife were breast feeding, I'd tell her, 'Don't breast-feed on one side more than the other."

He says he can't offer much more in the way of advice, except, "There a good deal of benefit in breast feeding. I'd almost always advise it."

Many medical authorities are concerned about the carcinogens in breast milk being consumed by infants. But most of them still believe that breast feeding's advantages far outweigh this concern.

Most of all, reflecting on his first, 1970 observation of the Hong Kong fisherwomen, Ing says: "Nature performs experiments. It performs experiments that we really can't do on people."

Medical science can't explain why some women get breast cancer and some do not. But Ing's Hong Kong observation, says Dr. Nicholas Petrakis of the University of California, may help medicine learn the mysterious mechanism.

Roy Ing was born in Hong Kong and went to the University of California at San Francisco Medical School.

It was in the summer of 1970, as a sophomore medical student on a summer research project, that he first encountered the Tanka fishing women and their lopsided breasts.

He was studying tuberculosis and the uneven breasts showed up on chest X-rays. The following summer, he recalls, back in Hong Kong as a cancer researcher for Dr. Petrakis, "I was struck by seeing four breast cancers among the fishing population. And all four had the cancer on the left side."

He finished medical school, became a Public Health Service doctor in Baltimore and in 1975 won a master's degree in public health from Johns Hopkins.

Then, still collabrating with Dr. Petrakis and still interested in the lopsided breasts and the suggestion of opposite-sided cancers, he returned to Hong Kong.

With the help of Dr. J.H.C. Ho at Queen Elizabeth Hosiptal, Ing now scrutinized the records of 2,372 women who had been treated for unilateral or one-sided breast cancer between 1958 and 1975.

The results: Among the whole group cancers were equally distributed between left and right breasts.

Among 39 Tanka fishwomen who developed a breast cancer before age 55, the division was also equal.

But among 27 Tanka who developed their breast cancers at age 55 or older, 79 per cent of the cancers had indeed 79 per cent of the left, unsuckled breast.

Petrakis and Ing know of no other female population who suckle their children only on one side. When Ing asked the fisherwomen why they did so, they said "because my clothes open on the right side" or "so I can used my other arm to feed myself," since they breast-feeed at family mealtime.

"But other women who breast-feed on both sides could say the same things." Ing says, "I think it's just a cultural thing, a strong cultural habit."

There is one research clue to what happens in unilateral feeding. Starting in the 1920s, report Drs. Ing. Ho and Petrakis in the June 16 Lancet, scientists began blocking the right or left breast of milk-giving mice, then giving the mice cancer-causing chemicals. They found that any cancers tended to concentrate in the unused breasts.

Petrakis and others have since found that harmful chemicals in food or the environment tend to accumulate in human ducts.

In short, Petrakis says: "If a woman lactates - gives milk - any chemicals she ingests promptly get into the milk. If she breast-feeds they pass out of the breast are gone. If the breast isn't used, it makes sense to assume they stay longer and eventually have some effect."

Would more breast-feeding, then, protect more women against cancer?

At one time doctors thought breast-feeding had a protective effect. Since 1970, as the result of studies in several countries, they have felt it has no effect on breast cancer development one way or the other.

"What we have now," said Petrakis, "is a discrepancy in that belief."

Neither Petrakis nor Ing can say just what to make of the Hong Kong finding, or whether it may indeed help explain how at least some breast cancer start.

But new medical thought is usually based on just such observations, and Petrakis credits this one to Ho and Ing - Ho because he had "the foresight to record some pertinent information" and Ing because he used his eyes.