We studied the use of a testicular hypothermia device worn daily for at least 16 weeks in 64 men with subfertile semen and elevated testicular temperature, who had had an infertile marriage for 2 or more years in which the wife was judged fertile. Improvement in 1 or more semen parameters was seen in 42 patients (65.6 per cent). Semen analysis was converted into the motile oval index, a numerical value representing the count, motility and normal morphology. The motile oval index helps to predict pregnancy outcome. Of 21 patients with pre-treatment motile oval indexes greater than 4.8 million per ml. 11 (52.4 per cent) produced pregnancy. Patients with lower starting indexes did not fare as well. Of 20 patients who met the criteria, and who wore the device for less than 2 weeks or not at all and had no other treatment 1 (5.0 per cent) produced pregnancy. Mean hypothermia time to date of missed menses was 4.2 months. Six patients with nonobstructive azoospermia showed no semen change with the testicular hypothermia device.