Thirty Klinefelter males treated with testosterone were studied by a follow-up examination carried out an average of 3.6 years after treatment began. The age of the men averaged 25.5 years at the time of the follow-up. Seventy-seven per cent of the men were judged to have benefited from testosterone treatment. They showed better mood, less irritability, more energy and drive, less tiredness, more endurance and strength, less need for sleep, better concentration ability and better relations with others during testosterone treatment. The beneficial effects of testosterone persisted in some of the men after cessation of treatment. The findings show that Klinefelter males given testosterone for the first time as adults can benefit from such treatment, even though it is preferable to start treatment at the age of 11-12 years.