OBJECTIVE To evaluate IDDM trends in Allegheny County, Pennsylvania, from 1965 to 1989. RESEARCH DESIGN AND METHODS A population-based registry established in 1976—the Allegheny County Registry—was used for this study through a retrospective chart review of all cases diagnosed in the county from 1965. Through this registry, cases of IDDM patients RESULTS For the first time a major increase in IDDM was seen (1985–1989). This increase in IDDM was heterogenous with the rise for males being double that for females. The increase for non-white males was considerably greater than that seen for white males, and an exceptionally large increase in the youngest age-group (0–4 yr of age) was observed. CONCLUSIONS Ecological analyses indicated an association between chicken pox infection and IDDM, and this suggested that at least part of the rise may be of an infectious origin.