BOSTON - Body builders already believe it, and science has finally proved it: Steroids make big muscles. But researchers found no evidence that steroids make users prone to outbursts of anger known as "'roid rage." The carefully controlled study showed convincingly for the first time that a few weeks of male sex hormone injections substantially beef up arms and legs and increase strength.

Men who exercised and took steroids for 10 weeks put on an average of 13 pounds of virtually pure muscle and could bench press an extra 48 pounds. In addition, psychological tests and questioning of the men's spouses found no evidence that steroids made them angrier or more aggressive.

Steroids are widely thought to cause extreme mood swings, and people charged with violent crimes have pleaded roid rage as a defense. But among steroid users who are mentally healthy, "testosterone doesn't turn men into beasts," said Dr. Shalender Bhasin of Charles R. Drew University in Los Angeles. Bhasin left open the possibility that in people who are mentally unbalanced to begin with, steroids can make them worse.

Bhasin and his colleagues said their results in no way legitimize steroid use by athletes. But they do suggest steroids might be a good way to help AIDS patients and others whose muscles waste away because of disease.

Possession and distribution of steroids without a prescription is a federal crime, punishable by up to a year in prison and a fine of at least $1,000. Doctors have warned that the potential side effects include sterility, testicular shrinkage, acne, abnormal liver function, baldness, high blood pressure and heart disease. In 1991, former football star Lyle Alzado publicly blamed steroid use for his inoperable brain cancer.

Despite the seemingly obvious evidence of weightlifters' bulging pecs, some doctors have doubted whether steroids really work. They argue that exercise, not injections, explains their muscles. "Intense debate on this issue has been raging for 30 or 40 years," Bhasin said. Earlier studies were flawed, in part because researchers gave only small amounts of steroids and failed to control the volunteers' exercise or diets.

To help settle the question, Bhasin and colleagues recruited 43 male volunteers and put them on a standard diet. They randomly assigned them to get either dummy shots or moderately high injections of testosterone enanthate, one of several anabolic steroids used by athletes.

Throughout the study, no one knew who was getting the real steroid shots. In both groups, half the men were either put on a weightlifting program or asked not to work out. The results, published in Thursday's issue of the New England Journal of Medicine, were clear and dramatic.

Those who took steroids but did nothing else improved their muscles and strength about as much or more than did those who exercised but got dummy shots. By far the most impressive change was seen in men who both exercised and got steroids. By the end of the experiment, men who got steroids but didn't exercise could bench press an extra 20 pounds, about the same as those who worked out but didn't get steroids. However, those who both took steroids and exercised could bench press an additional 48 pounds, a 23% increase.

Those who got steroid shots but did not exercise gained seven pounds of fat-free mass, which is mostly muscle, compared with an extra four pounds in those who exercised without steroids. Men who both exercised and got steroids put on 13 pounds.

The researchers saw similar differences in the size of the men's thigh and forearm muscles and in their ability to lift in squatting exercises. "The major implication is not to rationalize the abuse of steroids by athletes," Bhasin said. "It provides a rationale for testing the idea that short-term, cautious use of testosterone may prevent muscle wasting in cancer, HIV, obstructive lung disease and other chronic illnesses."

In the study, the men took weekly injections of 600-milligram doses for 10 weeks. This gave them about six to eight times more testosterone than their bodies produced naturally. Dr. C. Wayne Bardin of the National Institute of Child Health and Human Development said the study also holds good news for athletes who don't resort to steroids. "It shows what a powerful stimulant to muscle growth exercise is," he said. "It ought to reassure athletes that if they exercise and train properly, they will get big muscles."