To test the hypothesis that insulin-binding antibodies (IBAs) appearing in the circulation before insulin treatment are markers of beta-cell damage, we studied the prevalence of IBAs and both conventional (IF-ICAs)-and complement (CF-ICAs)-fixing cytoplasmic islet cell antibodies in 60 newly diagnosed diabetic children with a mean age of 9.5 yr. Seventeen (28.3%) had an insulin binding exceeding the upper range (2.8%) of that observed in 68 age-matched controls. The IBA-positive subjects were characterized by a younger age of onset [6.2 +/- 4.0 (SD) vs. 10.8 +/- 3.2 yr; P less than 0.001], lower glycosylated hemoglobin A1 levels (14.1 +/- 3.1 vs. 16.0 +/- 3.0%; P less than 0.05), lower serum C-peptide concentrations (0.12 +/- 0.07 vs. 0.20 +/- 0.17 nmol/L; P less than 0.05), and an increased frequency of HLA-Dw4 (9/13 vs. 11/37; P less than 0.05). There was no significant relation between IBAs and serum C-peptide concentrations after age adjustment by multiple regression analysis. Forty-three children (75%) were positive for IF-ICA and 38 (63.3%) for CF-ICA. Twelve IBA-positive diabetics (70.6%) had IF-ICA as well as CF-ICA in their serum. No association could be observed between IBA and either IF-ICA or CF-ICA, however. The results suggest that IBAs developing before diagnosis serve as an indicator of clinical and genetic heterogeneity within IDDM rather than as a marker of autoimmune beta-cell destruction.