Adult male residents of 13 counties of western Washington state in whom testicular cancer had been diagnosed during 1977-1983 (n = 333) were interviewed over the telephone regarding their history of genital tract conditions, including vasectomy. For comparison, the same interview was given to a sample of 729 men selected from the population of these counties by dialing telephone numbers at random. A higher proportion of cases than controls reported having had a vasectomy (relative risk = 1.5, 95 per cent confidence interval = 1.0-2.2). However, the association was restricted entirely to Catholic men. Whereas a history of vasectomy was reported with approximately equal frequency by Catholic and non-Catholic cases, only 6.3 per cent of Catholic controls reported such a history in contrast to 19.7 per cent of other controls. While the authors cannot rule out the possibility that there is a true difference of the effect of vasectomy on the incidence of testicular cancer as a function of religion, it seems more plausible that selective underreporting by Catholic controls has produced a spurious relation.