Incidence of polymyositis-dermatomyositis: a 20-year study of hospital diagnosed cases in Allegheny County, PA 1963-1982. The incidence of hospital diagnosed polymyositis-dermatomyositis (PM-DM) among residents of Pittsburgh and Allegheny County, PA was determined from 1963 to 1982. Medical records of all Allegheny County hospitals were searched for patients diagnosed PM-DM, and each accepted case was reviewed and verified by a physician, according to specified criteria. Direct age adjusted incidence rates were computed for race and sex, as new cases/million population/year, with the 1970 Allegheny County population as the standard. One hundred seventy-seven cases were identified during this 20-year survey for a total annual incidence of 5.5/million population. Incidence more than tripled during 1973-1982 compared to the first decade of study, with the greatest increase occurring in black women. Overall, the female to male incidence ratio was 2.2:1, but during the childbearing years (ages 15-44) this ratio increased to greater than 5:1 in both races. Detection of PM-DM is increasing, as shown by temporal trends. Although our series has significantly more adult PM and less DM and overlap cases than other published studies, annual incidence is similar to that reported in other locales. 