The incidence of insulin dependent diabetes mellitus, as determined from the Pittsburgh Insulin Dependent Diabetes Mellitus Registry, was examined in relationship to social class. The registry consists of all Allegheny County, Pennsylvania, cases of insulin dependent diabetes diagnosed under age 20 years in 1965-1976. Analyses of the registry data revealed little relationship between incidence, age at onset and social class. The lack of a strong relationship to social class indicates that the epidemiology of insulin dependent diabetes is not similar to those of polio or Hodgkin's disease, in which social class is strongly related to incidence and age at onset.