Seventy-seven couples in whom conventional in-vitro fertilization (IVF) had remained unsuccessful because of low fertilization rate and abnormal sperm characteristics were given either testosterone undecanoate 120 mg/day, or placebo during 3 months, after which a new IVF treatment was applied under identical technical conditions. There were no significant changes in sperm characteristics among the treated and placebo couples and the fertilization rate showed a similar increase after treatment in both groups. No significant difference in pregnancies occurred, with 32% pregnancies in the placebo controls and 17% among couples treated with testosterone undecanoate. It is concluded that testosterone undecanoate intake does not improve sperm characteristics, or the in-vitro fertilizing potential, or pregnancy rate over those observed in the placebo controls in cases with primary idiopathic testicular failure.