Treatment with 120 mg/day of Andriol (testosterone undecanoate; Organon, The Netherlands) was given to 11 men whose semen had either failed to fertilize, or had resulted in a less than 33% fertilization rate in a first in-vitro fertilization (IVF) trial. Repeat IVF at the end of a 3 month treatment period resulted in a highly significant increase in the number of oocytes fertilized from 4/95 (4.2%) before to 23/87 (26.4%, P less than 0.001) after Andriol treatment. One couple attained spontaneous conception during the second month of Andriol intake and three pregnancies occurred among the remaining 10 cases undergoing repeat IVF, for a total ongoing pregnancy rate of 36.4%. Sperm concentration but not motility increased in the native semen after Andriol intake, but there were no significant changes in sperm characteristics after preparation. The improvement in pregnancy rate contrasts favourably with the results obtained by repeat IVF of untreated historical controls. The result obtained in the present pilot study should encourage the performance of a larger, placebo-controlled investigation protocol.