Objective: To examine prospectively the relationship between vasectomy and prostate cancer.
Design: Cohort study.
Setting: Health professionals (dentists, veterinarians, osteopaths, optometrists, pharmacists, and podiastrists) in the United States.
Participants: There were 10,055 male members of the Health Professionals Follow-up Study, aged 40 to 75 years, who had had a vasectomy, and 37,800 members who had not had a vasectomy at the time of study entry in 1986. These participants had provided detailed information on various life-style variables including diet.
Main outcome measure: Diagnosis of prostate cancer.
Results: Between 1986 and 1990, 300 new cases of prostate cancer were diagnosed in participants who were initially free of diagnosed cancer. Vasectomy was associated with an elevated risk of prostate cancer (age-adjusted relative risk, 1.66; 95% confidence interval, 1.25 to 2.21; P = .0004). This elevated risk persisted after excluding 21 stage A1 cases (age-adjusted relative risk, 1.56; 95% confidence interval, 1.15 to 2.11; P = .004). Among men who had their vasectomy at least 22 years in the past (before 1965), the risk of prostate cancer was even higher (relative risk, 1.85; 95% confidence interval, 1.26 to 2.72; P = .002). This elevated risk among men with vasectomy did not appear to be caused by detection bias and persisted when we controlled for diet, level of physical activity, smoking, alcohol consumption, educational level, body mass index, and geographical area of residence.
Conclusions: These results support evidence from other epidemiologic studies that vasectomy increases risk of prostate cancer. The consistency of results among various epidemiologic studies, the increase of risk over time following vasectomy, the apparent lack of confounding or bias, and the existence of physiological changes in the prostate following vasectomy suggest that the association may be causal.