Scrotal temperature was monitored using a portable data recorder for periods of 24 h in six normal volunteers and 48 infertile patients with unilateral varicocele while subjects pursued their regular daily activities. Temperatures during sleep (TS) were generally higher than daytime values (TD), probably as a consequence of thermal insulation in bed. These diurnal variations were found to be less pronounced in the infertile patients than in volunteers (TS - TD = 0.29 degrees C +/- 0.06 degrees C vs 0.88 degrees C +/- 0.12 degrees C; P < 0.01). Moreover, scrotal temperatures at night of patients and volunteers were indistinguishable statistically, but were different during daytime hours. After successful ligation or embolization of the spermatic-vein in 16 patients, no change in scrotal temperature was observed. Although sperm counts were higher after treatment, this difference was not significant (67.4 +/- 17.2 x 10(6) vs 105.8 +/- 25.5 x 10(6); P > 0.05). The data support the view that varicocele-related damage to the testis results from a lack of adequate cooling, and that treatment does not normalize the temperature pattern.