Autoimmune diseases are among the leading causes of death among young and middle-aged women in the United States. Incidence rates vary among the autoimmune diseases, with estimates ranging from less than one newly-diagnosed case of systemic sclerosis to more than 20 cases of adult-onset rheumatoid arthritis per 100,000 person-years. Prevalence rates range from less than 5 per 100,000 (e.g. chronic active hepatitis, uveitis) to more than 500 per 100,000 (Grave disease, rheumatoid arthritis, thyroiditis). At least 85% of thyroiditis, systemic sclerosis, systemic lupus erythematosus, and Sjögren disease patients are female. Although most diseases can occur at any age, some diseases primarily occur in childhood and adolescence (e.g. type 1 diabetes), in the mid-adult years (e.g. myasthenia gravis, multiple sclerosis), or among older adults (e.g. rheumatoid arthritis, primary systemic vasculitis). Ethnic and geographic differences in incidence of specific autoimmune diseases have been documented, but specific groups may be at higher risk for some diseases and lower risk for other diseases. The incidence of type 1 diabetes increased but the rates of rheumatoid arthritis declined over the past 40 years. Thus although there are commonalities, there are also important demographic differences between diseases. Disease-specific research, as well as studies that focus on potentially related diseases, needs to be conducted.