All newly diagnosed diabetic patients in Sweden aged 15-34 years have been registered since 1983. In this study the clinical characteristics initially and after 2.5-3 years were evaluated by a questionnaire to the patient's physician and by non-fasting C-peptide. The study comprised patients registered 1983-84, and for 281 patients (37%), complete information was obtained. At diagnosis 75% were classified as Type 1, 19% as Type 2, and 6% as secondary diabetes or as uncertain by their physician. Twenty patients (7.1%) were reported to have ketoacidosis. Seventy-five percent were treated with insulin, 7% with oral hypoglycaemic agents (OHG), and 18% with diet alone. At follow-up 71% were classified as Type 1, 21% as Type 2, and 8% as secondary or uncertain while treatment was 82% insulin, 8% OHG, and 9% diet. During the follow-up period 42% of the initially non-insulin-treated patients were put on insulin whereas only a few stopped insulin treatment. Patients treated with diet or OHG at follow-up were older, had higher percent desirable weight, and lower blood glucose at diagnosis than patients treated with insulin. All except one patient had measurable random C-peptide at follow-up and mean values were for patients treated with insulin 0.55, OHG 1.41 and diet alone 1.29 nmol l-1. Random blood glucose results were similar. In conclusion the majority of newly diagnosed patients in the age group 15-34 years have the characteristics of Type 1 diabetes and Type 2 diabetes is rare before 25-30 years of age.(ABSTRACT TRUNCATED AT 250 WORDS)