Anal cancer is a rare tumour in Britain and its epidemiology has not previously been studied in this country. Several studies from the United States have shown an association between single marital status at the time of tumour registration (as a marker of male homosexuality in these populations) and the incidence of anal cancer. This study has used registry information on martial status for anal cancer and for colon cancer (controls) from the Thames, West of Scotland and West Midlands Cancer Registries. The registry data on marital status was validated using death certificate information. The relative risk of developing anal cancer was found to be significantly increased in single men for all three registries individually and for the combined data sets (OR 2.2' 95% CI 1.8-2.8). This accords with the findings of similar studies in the United States and supports the hypothesis that a sexually transmissible agent may be involved in the aetiology of anal cancer. For women, being unmarried was found to be protective against anal cancer in the combined data sets (OR 0.6; 95% CI 0.5-0.8).