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Summary Hormonal male contraception based on testosterone alone or on a combination of testosterone with a gestagen has been shown to suppress spermatogenesis effectively and to be fully reversible. However, clinical studies to date have only included volunteers with so-called 'normal' semen values by WHO standards. As a male contraceptive should be available to all interested men regardless of their semen parameters, we investigated how volunteers with subnormal semen parameters would respond to hormonal male contraception. During a 34-week treatment phase, the volunteers received injections of 1000 mg testosterone undecanoate in weeks 0, 6, 14 and 24. This was followed by a 24-week recovery and follow-up period. As it was not known whether men with subnormal semen parameters would recover to starting levels, cryopreservation of semen was offered to all subnormal volunteers. Twenty-three men with normal semen parameters and 18 with sperm counts below 20 million completed the trial. The normal volunteers showed the expected response with 17 suppressing sperm counts below 1 million/ejaculate (13 showing azoospermia) and six not-suppressing below 1 million sperm/ejaculate. By the end of the recovery period, all sperm counts had returned to the range of starting values. The subnormal group showed a similar pattern with 13 of 18 (= 72%) men suppressing below 1 million/ejaculate (8/18 = 44% showing azoospermia) and the remaining 5 of 18 (= 28%) not-suppressing sperm counts below 1 million/ejaculate. All sperm counts returned to the starting range. The study shows that in Caucasian men with normal sperm counts as well as in men with subnormal sperm counts, testosterone alone can produce azoospermia in about half and suppression below one million in about two-thirds of the volunteers. The same proportion of men in both groups appears to require an additional gestagen for full contraceptive protection. Most importantly, regarding suppressibility and reversibility, volunteers with normal and subnormal sperm counts display the same pattern. [ABSTRACT FROM AUTHOR]