OBJECTIVE To investigate the morphological changes of spermatozoa in the proximal vas deferens after vasectomy. METHODS Proximal vas deferens fluids were collected from 79 fertile men (group A) and 64 vasectomized men (group B) during the operations of vasectomy or vasovasostomy. Sperm morphology in the proximal vas deferens was analyzed after staining with the modified Papanicolaou method. RESULTS The percentage of spermatozoa with a normal oval head from group B (50. 7% +/- 21.7%) was significantly lower than that of group A (75.2% +/- 11.1%). The data in group A was similar to those of normal semen and therefore represents the physiological condition of the proximal vas deferens sperm of fertile men. There were no significant differences in the percentages of normal oval heads in group B with the time since vasectomy. CONCLUSIONS After vasectomy, the spermatozoa in the proximal vas deferens and epididymis were continuously degenerating and being replenished by spermatozoa coming from testis. The obvious morphological degeneration occurred in the sperm tail and spermatozoa with a small oval head and amorphous heads were increased.