The clinical characteristics, autoantibody profiles and seroprevalence of human T lymphotropic virus Type 1 (HTLV-1) were assessed in 30 Jamaican patients with Type 1 diabetes mellitus. Two hundred and fifty-two blood donors and 108 patients with Graves' disease were included as controls for the HTLV-1 component of the study. The mean age of onset of diabetes mellitus was 20.5 +/- 9.2 years and the mean duration of diabetes mellitus was 10.5 +/- 6.1 years. The remarkable clinical data included an absence of other associated organ-specific autoimmune diseases, and clinical evidence and history of congenital rubella in one patient. Islet cell cytoplasmic antibodies (ICA) were absent but 17% (5/30) of the diabetic patients tested positive for glutamic acid decarboxylase (GAD) antibodies. No other organ-specific autoantibodies were detected but non-organ-specific autoantibodies were present in 9 (30%) of the sera of diabetic patients. The seroprevalence of HTLV-1 in the patients with diabetes mellitus was significantly higher than that in the healthy controls (17% (5/30) versus 4% (11/252), p = 0.05). Autoantibodies were found in the sera of 4/5 (80%) of the diabetic patients who were positive for HTLV-1. None of the patients with onset of diabetes mellitus below age 15 years was HTLV-1 positive. The likely polyaetiological nature of Type 1 diabetes mellitus in Jamaicans is being further investigated at the molecular level.