Intrascrotal temperatures were measured bilaterally by a non-invasive method in 300 subfertile men (mean sperm count 21.4 x 10(6)/ml) and 30 normospermic control men (mean sperm count 118.7 x 10(6)/ml). The subfertile men had mean (s.d.) temperatures of 34.7 degrees C (0.8) for the right and 34.8 degrees C (0.7) for the left testis. The value for both testes of the control men was 33.4 degrees C (0.6). The difference (1.3-1.4 degrees C) was significant (P = 0.03). An intrascrotal temperature of greater than 34.1 degrees C was found in greater than 83% of subfertile men, regardless of clinical diagnosis. This method can therefore be used to survey large numbers of men. We suggest that small intrinsic temperature increases may interfere with the ability of the testis to accommodate to environmental temperature stresses and so lead to abnormal semen and subfertility.