Vasectomy techniques and failure rates vary among surgeons, and the criteria for failure are not often clearly defined. To help establish a yardstick for comparative purposes, a series of 8879 consecutive vasectomies performed with uniform technique over 24 years was reviewed. A subgroup of 5331 men who had returned for at least two postoperative semen tests--the study group--was used for follow-up analysis. Failures were defined as early or late and also were categorized as overt or technical according to the numbers, motility, or persistence of the remaining spermatozoa. There were 97 failures of all types, including 32 (0.60%) early and overt failures and 61 (1.14%) technical failures that involved the persistence of small numbers of spermatozoa, possibly of no significance. Four (0.08%) late overt failures were also seen; each of these was discovered as a result of a pregnancy, and each occurred at least four years after two azoospermic test results. Of the 97 failures, four were recognized as due to missed vasa deferentia, and the remainder were attributed to recanalization. Whether improved and reproducible failure rates can be consistently obtained by other techniques is not yet clear.