We investigated the intercellular adhesion molecule-1 (ICAM-1) gene polymorphism in 90 patients with young-onset type 1 diabetes, 74 with adult-onset type 1 diabetes, and 171 control subjects. The distribution of C-T genotypes and allele frequencies in exon 6 of the ICAM-1 gene was significantly different between adult-onset type 1 diabetes patients and controls (chi(2) = 9.76, p = 0.0076), and between patients with adult-onset and young-onset type 1 diabetes (chi(2) = 11.28, p = 0.0036). In contrast, we failed to detect any association between patients with young-onset type 1 diabetes and controls. Our data suggest that ICAM-1 exon 6 gene polymorphism affects the age-at-onset of type 1 diabetes and that different pathogenetic mechanisms may exist between young-onset and adult-onset type 1 diabetes.