The results of semen examination in 313 vasectomized males are presented. A quantitative analysis of the semen samples was performed according to the WHO criteria and the centrifugate of the entire ejaculate was studied. At the first semen examination, which was performed three months after the procedure, the sperm count per ml was negative in 210 patients (83.7%), positive (all non-motile and dead) in 41 (16.3%). Sixty-two patients did not send a sample. Those cases with a positive first examination had a regular examination every two months. At five months 90.8% of these cases had a negative count per ml. 3.6% of the patients required 7-10 months to have a negative count. It took longer for the older patients to have a negative count, the difference being statistically significant with respect to the younger patients. Examination of the centrifugate revealed spermatozoa in 24.3% of the samples with a negative sperm count per ml, although these were all non-motile and dead, except for one case who had motile spermatozoa at 3 months. There were no pregnancies or revasectomies. Some legal aspects relative to patient examination and follow-up are discussed.