Vasectomy and Prostate Cancer

This issue ofThe Journalincludes two reports of cohort studies by Giovannucci et al that find an association between vasectomy and subsequent risk of prostate cancer.¹,²Both vasectomy and prostate cancer are common and any causal relationship between the two would be important both for individual and public health. Although the studies by Giovannucci et al are of high quality, whether the relationship they report on is causal remains unclear. The age-adjusted relative risks for prostate cancer among men with vasectomy in the Nurses' Health Study and the Health Professionals Follow-up Study were 1.56 (95% confidence interval, 1.03 to 2.37) and 1.66 (95% confidence interval, 1.25 to 2.21), respectively. When risk estimates of this magnitude are derived from observational studies, they are considered to be weak associations that may be due to chance, bias, or a causal relationship.³It is our See also pp 873 and 878.