Objective To quantify germ cell loss and the extent of testicular fibrosis in vasectomized patients of varying obstructive intervals. Design:Retrospective study. Setting Specialized male reproductive surgery and endocrinology service. Patient(s) Thirty-four vasectomized patients 1-20 years after surgery and 10 normal subjects. Intervention(s) Thirty-four testicular biopsies taken at the time of vasectomy reversal (vasovasostomy). Control biopsies taken from 10 normal men at the time of vasectomy. Main outcome measure(s) Stereological assessment of testicular germ cell populations and testicular fibrosis. Result(s) Vasectomy caused a significant decrease in germ cells in the later stages of spermatogenesis, with significant reductions in pachytene spermatocytes (by 18%), round spermatids (by 40%), elongating spermatids (by 23%), and elongated spermatids (by 39%). The loss of spermatids showed a significant relationship with obstructive interval. A significant 2.7-fold increase in total (peritubular plus interstitial) fibrosis was observed, which showed a positive relationship with obstructive interval. Decreased germ cell populations and fibrosis did not seem to be related to sperm antibody levels nor to sperm counts obtained up to 2 years after vasovasostomy. Conclusion(s) Vasal obstruction results in significant reductions in germ cells in the later stages of spermatogenesis and increases in testicular fibrosis, both worsening with an increasing obstructive interval. Testicular damage after vasectomy might impact upon the prospects for reversal.