The main purpose of this retrospective, cross-sectional study was to evaluate the effectiveness of vasectomy in an ongoing public sector program in Nepal. We evaluated semen samples from men who had previously had a vasectomy, and asked about the occurrence of pregnancies in the men's partners. In addition, the surgeons who performed the vasectomies completed a questionnaire about their techniques. A two-stage stratified sampling procedure was used to select 1263 men from among over 30,000 men, who had previously undergone a no-scalpel vasectomy, mostly by ligation and excision, in 32 districts between July 1996 and June 1999. Semen samples were preserved and analyzed at a central laboratory. A US andrology laboratory validated the lab results. Twenty-three men (2.3%, 95% confidence interval [CI] 1.1-3.6) had >/=500,000 sperm/mL in their semen. Fifteen of those men reported pregnancies conceived after their vasectomy. In addition, six men with azoospermia reported pregnancies for which conception occurred within 3 months after vasectomy. Eleven men with azoospermia reported pregnancies for which conception occurred more than 3 months after vasectomy. Reported pregnancy was more likely in younger partners. The life table pregnancy rates for all men interviewed were 0.7 (95% CI 0.2-1.1), 1.7 (95% CI 1.4-2.1) and 4.2% (95% CI 3.2-5.2) at 3, 12 and 36 months, respectively. In low-resource, programmatic settings, vasectomy failure rates may be higher than commonly cited rates, especially in younger populations. Additional research is needed to determine if other occlusion techniques could reduce failure rates. Counseling on vasectomy should always convey the possibility of failure and partner pregnancy.