The study was too small and short-term to address another longstanding question about testosterone gels: whether using them increases the risk of heart disease and prostate cancer and other conditions. And that, said Dr. Richard J. Hodes, the director of the National Institute on Aging, was deliberate.

About 15 years ago, worried about soaring numbers of men using testosterone gels, Dr. Hodes and administrators at the Department of Veterans Affairs suggested a huge clinical trial that would involve thousands of men, go on for years and find out definitively what the risks and benefits of testosterone treatment were. But the plan ran into resistance.

There were real hazards to men participating, critics said. Not just risk of cancer and, it later emerged, the possibility of heart disease, but an issue with P.S.A. tests, the blood examinations used to screen for prostate cancer. Testosterone increases P.S.A. levels. Men with high levels usually get biopsies of their prostates to look for cancer. But in a large clinical trial, investigators do not know who is getting the drug and who is getting a placebo. So thousands of men taking testosterone could end up with biopsies resulting solely from P.S.A. levels that were raised by the drug, not by cancer.

Dr. Hodes turned to the Institute of Medicine for guidance. A group of medical experts there advised starting small. Do a study, they said, that first asks if there is any benefit to testosterone in older healthy men with low levels of the hormone. If there is no benefit, why do a trial?

The new study is the result. The testosterone gels used by the men in the study are not as powerful as the very high doses of testosterone and similar hormones that some bodybuilders and athletes have injected to grow muscle and improve performance.

Some men with serious medical conditions that deplete their bodies of testosterone use the hormone as therapy and that practice is not in question, researchers said. At issue is the men whose testosterone levels dropped simply because they grew older.

For David Bostick, a 71-year-old Pittsburgh man who participated in the study, the appeal of testosterone was that it might help with his sluggish feeling and reduced libido. After a few weeks of smearing a gel — he did not know if it was AndroGel or a placebo — on his stomach, he began to feel different, with more sexual desire and more energy. He guessed he must be receiving testosterone. When the study ended, he saw his primary care doctor and obtained a prescription for AndroGel. He knew there were possible risks but, he said, “I made an informed decision to take it.” On Saturday, he received a letter from the study finally telling him what was in the gel he had used. It was testosterone.