To assess the clinical significance of subclinical right varicocele, we used scrotal sonography for a prospective study of 68 infertile patients with a clinical left varicocele. A significant difference was found in the improvement in postoperative semen characteristics between sonographically diagnosed bilateral and unilateral groups. We conclude that a subclinical right varicocele is also detrimental for spermatogenesis in patients with a clinical left varicocele.