The etiology of mycosis fungoides is obscure, and the risk factors for its occurrence are poorly documented. This investigation uses data from nine US population-based cancer registries to investigate the descriptive epidemiology of this disorder. From 1973 through 1984, 721 newly diagnosed cases of mycosis fungoides were reported to these registries (0.29 cases per 100 000 population per year). A dramatic increase in the incidence of mycosis fungoides was noted over the period of this study. The incidence was highest among the elderly. Blacks were twice as likely to be afflicted as whites, and the incidence among men was more than twice the incidence among women. The geographic variation in incidence was associated with several demographic variables, including population density, family income, and concentration of physicians. Analysis of mortality among these patients revealed no evidence of detection bias.