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 less than 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 less than 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 less than 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 less than 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.