During a period of four years, 386 patients with left sided varicoceles were treated by sclerotherapy of the left internal spermatic vein. This was successful in 93.8%. The most common complication was perforation of the vein in 6.2%. Technical success and complications depended on anatomical variations. Of particular significance was Bähren type IVb (competent main vein, incompetent collaterals) which occurred in 18.9%. This accounted for 66.5% of all perforations and 54.2% of unsuccessful interventions. The recurrence rate was 3.8%.