Fifty-nine patients were diagnosed with diabetes in the ten years from 1987 to 1996 in the Division of Endocrinology and Metabolism, Department of Pediatrics, Faculty of Medicine Siriraj Hospital, Bangkok, Thailand. All patients were less than fifteen years old. Fifty-five patients (93.3%) were type 1 diabetes, three (5%) were type 2 diabetes and MODY, and one (1.7%) was diabetes secondary to beta thalassemia major. Patients with type 2 diabetes, MODY, and secondary diabetes were excluded from this study, and fifty-five patients with type 1 diabetes were analysed. The aims of this study were to determine some of the general characteristics of Thai childhood type 1 diabetes and to see whether a seasonal variation is present. The results showed a female to male ratio of 1.39:1. The peak age at diagnosis was from 9 to 12 years. Seventy-seven per cent had diabetic ketoacidosis at the time of diagnosis. The majority of patients (93.9%) had a BMI of less than 20 kg/m2. A family history of diabetes was reported in 38 per cent but only 2 per cent were type 1 diabetes. We found a high prevalence of patients in the summer and winter seasons (35-48% and 37-50% respectively) and a lower prevalence in the rainy season (14.8-15%). These results are different from a previously reported study in 1984-1985 which found no differences in summer, winter, and rainy seasons. Further research study into Thai childhood type 1 diabetes is needed, especially the influence of seasonal factors, the incidence of the disease, and the significance of family history.