Limited data are available on the possible changes in cancer risk brought about by widowhood and divorce, an increasing segment of the population. We calculated standardized incidence ratios (SIRs) for cancer among 47,000 widows/widowers and 60,000 divorced people, based on the Swedish Family-Cancer Database. Persons had to be identified with the same civil status in the census of years 1960 and 1970; the comparison group was married people according to the same censuses. Cancers were followed from years 1971 to 1998. Both increased and decreased SIRs were found, and a consistent pattern emerged. The effects on the divorced were always stronger than those in widows/widowers, irrespective of the direction of the effect. Every significant SIR for a cancer site in widows/widowers was accompanied by a more deviant and significant SIR in the divorced. SIRs between divorced men and women (r=0.83, P<0.0001) and between widows and divorcees correlated (r=0.70, P<0.0001). The overall cancer risk for the divorced was 0.92-0.94, and it was a balance between increased risks at tobacco-, alcohol-, and human papilloma virus-related sites, and decreased risks at most other sites. The data suggest that the changes in lifestyle on the loss of a spouse impact on the incidence of almost every type of cancer. The effects were so large that a failure to consider marital status in epidemiological studies may be a source to bias. Understanding these lifestyle changes may provide new insight in cancer prevention.