Who is at risk?

Polymyositis is a relatively rare disease. Women are more likely to have it than men. Although people of any age can get it, it most commonly affects people between the ages of 20 and 60 years. Children have been known to develop dermatomyositis.

Estimates of incidence range from 0.5 to 8.4 cases per million. The age at onset peaks between ages 10 and 15 years in childhood disease and between ages 45 and 60 years in adults. Inclusion body myositis is more common after age 50. In general, women are affected twice as often as men; however, inclusion body myositis affects twice as many men. In adults, lowest rates are reported in the Japanese and the highest in African Americans.