Aim: To evaluate the treatment outcome of antegrade internal spermatic vein sclerotherapy in men with non-obstructive azoospermia or severe oligoteratoasthenospermia (OTA) as a result of varicocele. Methods: Between September 1995 and January 2004, 47 patients (mean age 33.8 ± 6.3 years) underwent antegrade internal spermatic vein sclerotherapy for the treatment of varicocele with azoospermia (14 patients) or severe OTA (33 patients). Testicular core biopsy was also performed in complete azoospermic patients who provided informed consent. The outcome was assessed in terms of improvement in semen parameters and conception rate. Results: Forty-two (89.4%) of 47patients had bilateral varicocele. Serum follicle stimulating hormone (FSH) did not differ between patients with azoospermia and severe OTA. After the follow-up of 24.8 ± 9.2 months, significant improvement was noted in mean sperm concentration, motility and morphology in 35 patients (74.5%). Comparison between groups during the follow-up revealed significantly higher values of sperm concentration, motility and normal morphology in the severe OTA group. Pregnancy was achieved in 14 cases (29.8%). Testicular histopathology of the azoospermic patients with postoperative induction of spermatogenesis revealed maturation arrest at spermatid stage, Sertoli-cell-only (SCO)with focal spermatogenesis or hypospermatogenesis. None of the patients with pure SCO pattern or maturation arrest at spermatocyte stage achieved spermatogenesis after the treatment. Preoperative serum FSH levels didn\'t relate to treatment outcome. Conclusion: Antegrade internal spermatic vein sclerotherapy is an easy and effective treatment for symptomatic varicocele. It can significantly reverse testicular dysfunction and improve spermatogenesis in men with severe OTA, as well as induce sperm production in men with azoospermia, improving pregnancy rates in subfertile couples.