CONTEXT: The risk of paternity after vasectomy is rare but still exists. Overall failure to achieve sterility after vasectomy occurs in 0.2 to 5.3% of patients due to technical failure or recanalization. The objective of this report was to describe a rare but notable case of proven paternity in which the semen analyses had not given evidence of spermatozoa. CASE REPORT: A 44-year-old vasectomized man whose semen analyses had shown azoospermia became a father four years after sterilization. Blood sample DNA analysis on the child and husband proved biological paternity. Vasectomy may fail in the long run even without spermatozoa in semen analysis. The patient must be aware of this possibility.