Percentages of various T-lymphocyte subpopulations in the blood were studied at the onset of Type 1 (insulin-dependent) diabetes. The number of lymphocytes with OKT8 markers was higher in the diabetic patients than in control subjects (p< 0.005) and the ratio between helper and suppressor/cytotoxic T-cells (OKT4/OKT8 ratio) was lower in the diabetic patients than in the control group (p< 0.005). The values in the diabetic patients were, however, essentially within the normal range. When Ia-antigen-positive cells were analysed in T-cell-enriched cell populations, Type 1 diabetic patients had higher percentages of these cells (p< 0.01), suggesting T-cell activation. When patients with either of the two major HLA risk antigens (Dw3 or Dw4) were compared, there was a significant difference in the OKT4/OKT8 ratio (p< 0.005), as Dw3-positive patients had higher and Dw4-positive patients lower ratios. This finding supports the concept of heterogeneity of the disease and can also explain the discrepant findings of earlier studies. When patients with complement-fixing islet cell antibodies were compared with patients without islet cell antibodies, there was no significant difference, although the OKT4/OKT8 ratio was slightly lower in the complement-fixing islet cell antibody-positive patients.