Epidemiologic studies of autoimmune diseases have not considered them in the aggregate. The objective was to estimate the prevalence of 30 autoimmune diseases separately and in aggregate according to ICD-10 classification. The lifetime prevalence of the entire population of 5,506,574 persons alive in Denmark on October 31, 2006, was estimated by linking records of all visitors to hospitals and specialty clinics via National Patient Registers from January 1, 1977 through October 31, 2006. The prevalences vary from 0.06/1,000 for Pemphigus to 8.94/1,000 for Type 1 diabetes. Nearly 4% of the population had one or more autoimmune disease. The general conclusion is that autoimmune diseases as an aggregate are common.