Improvement of sperm qualities and fertility after operation for varicocele is well-documented. The effect does not appear to relate to the degree of dilatation of the spermatic veins. For this reason 22 men with severely impaired fertility but without any clinical sign of varicocele were offered operation, i.e. ligation of the left spermatic vein(s). Fertility was evaluated by repeated sperm examinations. On average, the childlessness lasted for 3.5 years before the operation took place. A sperm examination was performed on average 6.3 months postoperatively. In half the cases improvement of sperm quality was found. In 9 cases no change was observed, and in one man the sperm quality was reduced. In 7 cases conception occurred within 16 months. The improvement of both sperm quality and the conception rate were highly significant in comparison with a non-operated control group with the same severely impaired fertility and without clinical sign of varicocele.