There is evidence from retrospective studies that varicocelectomy can improve sperm DNA damage in infertile men with a clinical varicocele. The objective of this prospective study was to examine further the effect of varicocelectomy on sperm chromatin and DNA integrity. We evaluated a consecutive series of infertile men (n = 25) who underwent microsurgical varicocelectomy for treatment of clinical varicocele. We examined conventional sperm parameters and sperm chromatin structure assay parameters (percentage DFI--DNA fragmentation index and percentage HDS--high DNA stainability, an index of chromatin compaction) before and 4 and 6 months after microsurgical varicocelectomy. Sperm DNA integrity improved significantly after surgery (percentage DFI decreased from 18 ± 11% before surgery to 10 ± 5%, and 7 ± 3%, at 4 and 6 months after surgery respectively). Sperm chromatin compaction also improved significantly after surgery (percentage HDS decreased from 11 ± 7% before surgery to 8 ± 6%, and 7 ± 5%, at 4 and 6 months after surgery, respectively). Sperm concentration and progressive motility improved after surgery, although the differences were not statistically significant when compared with that before surgery. The data show that varicocelectomy is associated with an improvement in sperm DNA integrity and chromatin compaction. These findings support the concept that correction of a varicocele can improve spermatogenesis, particularly spermiogenesis (the stage in spermatogenesis where compaction and stability of the sperm DNA and chromatin occur).