Background: Persons being treated with IFNα-2b for advanced cutaneous malignant melanoma (CMM) have been reported to have a greatly improved prognosis if they develop autoantibodies or clinical signs of autoimmunity during therapy. Consequently, we examined whether autoimmune diseases might also be associated with lower CMM incidence and better prognosis. Methods: We established a study cohort based on the entire Danish population, obtaining information on CMM and autoimmune diseases from the Danish national registers. Using log-linear regression models adjusting for age, period, and sex, we compared CMM incidence and CMM-specific mortality rates in persons with and without a history of autoimmune disease. Results: Between 1977 and 2003, 20,482 cases of CMM were registered in the Danish Cancer Register. Previously diagnosed autoimmune diseases did not affect the incidence of CMM (incidence rate ratio, 1.0; 95% confidence interval, 0.9-1.1). In the first 5 years after CMM diagnosis, we observed 8,957 deaths in individuals with CMM (5,181 expected). CMM-specific mortality rates 1 to 5 years after diagnosis were similar in CMM patients with and without autoimmune diseases (mortality rate ratio, 0.9; 95% confidence interval, 0.7-1.2). Conclusions: Autoimmune conditions were not associated with CMM incidence or prognosis. The better CMM prognosis previously observed when autoantibodies or clinical signs of autoimmunity developed during IFNα-2b therapy may have been related to variation in individual responses to this therapy, with individuals sensitive to treatment exhibiting more signs of autoimmunity but also (independently) experiencing greater antitumor responses as a result of treatment.