Purpose: We prospectively evaluated changes in sperm chromatin structure in infertile patients before and after surgical repair of varicocele, and the impact on the pregnancy rate. Materials and Methods: Included in the study were 49 men with at least a 1-year history of infertility, a palpable varicocele and oligospermia. World Health Organization semen analysis and sperm DNA damage expressed as the DNA fragmentation index using the sperm chromatin structure assay were assessed preoperatively and postoperatively. Pregnancy (spontaneous and after assisted reproductive technique) was recorded 2 years after surgery. Results: Mean sperm count, sperm concentration and sperm progressive motility improved significantly after varicocelectomy from 18.3 x 10(6) to 44.4 x 10(6), 4.8 x 10(6)/ml to 14.3 x 10(6)/ml and 16.7% to 26.6%, respectively (p < 0.001). The DNA fragmentation index decreased significantly after surgery from 35.2% to 30.2% (p = 0.019). When the definition of greater than 50% improvement in sperm concentration after varicocelectomy was applied, 31 of 49 patients (63%) responded to varicocelectomy. Conclusions: After varicocelectomy sperm parameters significantly improved and sperm DNA fragmentation was significantly decreased. Low DNA fragmentation index values are associated with a higher pregnancy rate (spontaneous and with assisted reproductive technique). We suggest that varicocelectomy should be considered in infertile men with palpable varicocele, abnormal semen analysis and no major female factors.