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°C ± 0.06°C us 0.88°C ± 0.12°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 × 106vs 105.8 ± 25.5 ± 106; 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.