A Negative correlation between spermatozoa output and serum gonadotropin levels, as well as between scrotal temperature and spermatozoa output, has been found in man. However, no studies have been done on the relationship between scrotal temperature and serum gonadotropin levels. This paper reports such data from 212 infertile men. The upper limit for normal scrotal temperature was defined as the 90th percentile value (35.3 C) of a control group of 64 fertile men whose mean serum FSH and LH levels were 6.0 +/- 0.8 (+/- SE) and 6.4 +/- 0.7 IU/L, respectively. This value for scrotal temperature (35.3 C) was used to classify infertile men into 3 groups: bilateral hyperthermia (n = 56), unilateral hyperthermia (n = 40), and bilateral normothermia (n = 116). In the unilateral and bilateral hyperthermic groups serum LH and FSH levels were significantly increased compared with those in the normothermic group. The mean serum testosterone values were similar in all groups. To study the relationships between serum gonadotropin levels or spermatozoa output and scrotal temperature, the infertile men also were divided into classes according to their spermatozoa output. These classes were subdivided into two groups, normothermic or hyperthermic, according to whether the left scrotal temperature was equal to or less than, or more than 35.3 C. For the infertile men whose spermatozoa output was more than 60 X 10(6) spermatozoa/ejaculate (normospermia), there was no significant difference between the serum gonadotropin levels of the normothermic (n = 42) and the hyperthermic (n = 20) groups. Among the oligospermic men (spermatozoa output, 0.1-60 X 10(6) spermatozoa/ejaculate), the hyperthermic group (n = 65) had significantly higher serum gonadotropin levels and significantly smaller testicular volumes than the normothermic group (n = 71). The two oligospermic groups also had significantly higher serum FSH values than the infertile normospermic groups. These results were not linked to the presence of a varicocele or a history of cryptorchidism, as the prevalence of varicocele and cryptorchidism was equally distributed within the groups studied. We conclude that the increase in serum gonadotropin levels in the case of a decrease in spermatozoa output is significantly greater in the presence of associated scrotal hyperthermia.