Cancer patterns among broad populations of homosexual men and women have not been studied systematically. The authors followed 1,614 women and 3,391 men in Denmark for cancer from their first registration for marriage-like homosexual partnership between 1989 and 1997. Ratios of observed to expected cancers measured relative risk. Women in homosexual partnerships had cancer risks similar to those of Danish women in general (overall relative risk (RR) = 0.9, 95% confidence interval (CI): 0.6, 1.4), but only one woman developed cervical carcinoma in situ versus 5.8 women expected (RR = 0.2, 95% CI: 0.0, 0.97). Overall, men in homosexual partnerships were at elevated cancer risk (RR = 2.1, 95% CI: 1.8, 2.5), due mainly to human immunodeficiency virus (HIV)/acquired immunodeficiency syndrome (AIDS)-associated Kaposi's sarcoma (RR = 136, 95% CI: 96, 186) and non-Hodgkin's lymphoma (RR = 15.1, 95% CI: 10.4, 21.4). Anal squamous carcinoma also occurred in excess (RR = 31.2, 95% CI: 8.4, 79.8). After exclusion of Kaposi's sarcoma, non-Hodgkin's lymphoma, and anal squamous carcinoma, no unusual cancer risk remained (RR = 1.0, 95% CI: 0.8, 1.3). With anal squamous carcinoma and HIV/AIDS-associated cancers as notable exceptions in men, cancer incidence rates among homosexual persons in marriage-like partnerships are similar to those prevailing in society at large.