Type I diabetes is an autoimmune disease characterised by a marked activation of peripheral T cells around the time of clinical diagnosis. Studies of T-cell antigen receptor Vp (TCRBV) gene usage in type I diabetes have been conflicting. Usng a semi-quantitative polymerase chain reaction technique and flow cytometry we have investigated the TCRBV gene usage of 13 newly diagnosed patients with type I diabetes and 11 normal healthy controls. No preferential TCRBV gene usage was found between patients and controls even after matching for HLA-DR3 and/or -DR4. In addition, no significant differences in TCRBV gene usage were found between sequential samples taken over a period of up to 7 months following diagnosis. These results suggest that the TCR repertoire of these patients is heterogeneous and it is unlikely that a single‘pathogenic’T-cell clone is dominant at the clinical onset of the disease.