In a large, representative sample of newly-diagnosed IDDM patients, using a highly sensitive assay to detect islet cell cytoplasmic antibodies (ICA), no marked differences were found between ICA + and ICA - patients on various clinical, genetic, immunologic, and epidemiologic characteristics. In particular, there was no evidence for associations between ICA status at diagnosis and either sex, race, family history of IDDM, HLA-DR phenotype, antibody titers to Coxsackie B viruses, immunoglobulin levels, C-peptide and glycosylated hemoglobin concentrations, or insulin requirements. The most significant relationship was between the presence of ICA and a young age at diagnosis; however, the large overlap between the distributions of the ages at onset for ICA + and ICA - groups on this variable suggests that this association is of limited importance. These data suggest that the presence or absence of ICA at diagnosis may not be useful in defining possible subtypes of IDDM.