OBJECTIVE: To evaluate serum leptin levels in children and young adults with type 1 (insulin-dependent) diabetes mellitus and to investigate whether they are different in prepuberty, puberty and young adulthood. DESIGN: Three groups of diabetics (prepubertal, pubertal and young adults) subdivided into obese and non-obese were studied. Three groups of healthy subjects matched for sex, age and body mass index served as controls. RESULTS: Diabetic patients had serum leptin concentrations similar to those of controls in all three groups. A small non-significant increase in leptin from the prepubertal to the young adult age group for both diabetics and controls was found. A significant association of serum leptin level with body mass index (P < 0.001), female sex (P < 0.001) and age (P < 0.01) in both the diabetic and control group was present. Insulin-dependent diabetes was not associated with higher leptin concentration. CONCLUSIONS: Serum leptin concentrations are similar in diabetic patients and healthy controls. The association between obesity and leptin concentration was similar in the diabetic and non-diabetic subjects. Type 1 diabetes mellitus does not modify serum leptin concentration.