In San Francisco, the crude incidence rate of squamous cell cancer of the rectum or anus in males averaged 11.3/million and 8.8/million in females in 1972-77. In the 4 other counties of the San Francisco-Oakland Standard Metropolitan Statistical Area, the rates were 3.8 in males and 7.3 in females. Fifty-five percent of the San Francisco male patients were single (i.e., never married) and, for the remaining counties, 18% were single. One-fifth to one-fourth of the adult population of San Francisco is believed to be homosexual. Thus a high rate of squamous cell carcinoma of the rectum or anus is associated with single males in a county widely noted for its homosexual community. This is consistent with a hypothesis that a venereally transmissable agent or its treatment is responsible for some portion of the incidence of that tumor.