PIP: To clarify the role of marital status in human carcinogenesis, a 1968 Cancer Institute study analyzed the cancer mortality experience of 31,658 white Catholic nuns from 41 religious orders in the U.S. from 1900-1954. The national white female population was used for cause-specific comparison and both groups were assigned cohorts depending upon the year of birth. When examined by 10-year age groups, rates for cancer at all sites was generally lower for nuns than for controls aged 59 or 69 but were substantially higher at older ages. Postmenopausal nuns (aged 69 and over) displayed a higher rate (38.6%) of cancer of the large intestine than did controls (22.6%) but had a lower proportion of deaths from cancer of the biliary passages and liver (13.0% vs. 22.6%). Nuns displayed a striking excess in breast cancer mortality over the age span of 40-74 years and had consistently higher rates than controls for each age group above 39 years. Lower cervical cancer rates for nuns (10.8%) than for controls (56.6%) seemed related to coital factors. Cancer of the uterus accounted for 63% of the genital cancer deaths among sisters. Overall, the genital cancer mortality rates for nuns were consistent with high mortality rates for the single, white female population of the U.S. The increased risk of breast cancer and cancers of the corpus uteri and ovary would seem to reflect an established link with infertility. Combination of these factors with the excess incidence of cancer of the large intestine among postmenopausal nuns suggests a common pathogenic mechanism of a hormonal nature operating in some women.