Smoking as few as three marijuana cigarettes a week over a long period may significantly harm the lungs, and harm them even more than tobacco cigarettes do, University of California scientists, reported yesterday.

The scientists' conclusion was based on observation of 74 regular marijuana smokers, men who have used the drug for more than five years, on the average.

Though their results strongly suggest the harmful effect, the researchers added, a study of hundreds of marijuana users should be made to make sure of it.

A federal marijuana reseacher, Dr. Richard Stillman of the National Institute on Drug Abuse, nonetheless called the California finding "new and significant."

"This effect has been suspected," Stillman said. "I would say very few people now believe marijuana is entirely innocuous, though the evidence is coming in rather slowly that it's not and there is still a whole spectrum of opinion ranging from saying it's harmless to saying it may be very harmful."

"What we are saying could be considered a caution," said Dr. Donald P. Tashkin, associate proffessor of medicine at the University of California at Los Angeles, who headed the Los Angeles-area study.

I'd say no one should be lulled into a false sense of security that smoking marijuana regularly - at least three times a week - is without any harmful effects on the lungs."

Tashkin, Barry Calvarese and Michael Simmons reported their study's results to an American Lung Association meeting in Boston.

Between 1973 and 1977, they studied the lung function of 74 men aged 21 to 33 who had used marijuana for at least two years, and smoked five marijuana cigarettes a week, on the average, for at least the previous six months.

Fifty of the 74 also smoked tobacco. But the scientists compared the marijuana users with a set of matched controls - non-marijuana users of similar ages and tobacco habits - to make sure they were not just seeing a tobacco effect.

On the average, the marijuana users' lung function, measured by how hard they had to breathe, was impaired by 25 percent.

"But this was small enough so the men weren't aware of it," Tashkin said. The men felt no shortness of breath and had no chronic coughs or disease.

Still, the lung effects were more marked in the chronic marijuana smokers than in men who smoked 16 or more cigarettes every day.

"We just don't know whether or not this impairment will lead to clinically significant disease," Tashkin added: Nor could the study tell whether or not the lung effects will be permanent.

"That's why we think it is urgent to do a long-range epidemiologic study - to follow at least 500 non-users in several cities over a period of years," Tashkin said.

Studies of affected lung tissue, too, should be made, he said. His theory is that the marijuana particles or gases "are probably causing some inflammatory changes in the large airways secondaryto the irritating effect of the smoke."