Fifty-six males from infertile couples were categorised into two different groups and their semen quality examined. Patients in which there was no evidence of interference with normal testicular thermoregulation either during the day or the night were classified as 'cool workers' and 'cool sleepers' (Group I, n = 26). In the other group (II), 'warm workers'/'warm sleepers' (n = 30), there was evidence for scrotal insulation. The number of good moving spermatozoa per ejaculate as well as this number per ml was greater in 'cool workers'/'cool sleepers' than in 'warm workers'/'warm sleepers' (p less than 0.001). These results emphasize the need for comprehensive patient history when evaluating infertility problems. Similarly, living habits may play a role in human male infertility and treatment.