IL-15 is a 14-15 kD cytokine produced by monocytes/macrophages and shares some biological actions with IL-2. The serum concentration of IL-15 in type 1 diabetic patients has not been reported seriously. Our studies were performed on 51 patients (28 women and 23 men) with type 1 diabetes mellitus. Healthy control subjects (n=22, 12 women and 10 men, mean age 29 years, range 24-32 years) were recruited from medical staff. IL-15 serum levels were detected by ELISA (R & D systems, USA). Short-term and long-term metabolic control parameters, lipid profile and C-reactive protein levels were also estimated. There was a statistically significant increase of serum IL-15 in type 1 diabetic patients in comparison to the control subjects (4.4 (1.5-11.8) versus 2.9 (1.5-6.0) pg/ml, p<0.05). Diabetic patients with higher IL-15 serum levels had higher HbA1c values. A correlation was found between IL-15 serum concentration and HbA1c (N(s)=0.31, p=0.029). There was no relation between acute hyperglycaemic episodes and IL-15 serum level. The potential associations between IL-15 serum level and long-term diabetic control lead us to speculate that IL-15 may serve as a target for future treatment in patients with prediabetes and/or for prevention of late diabetic complications.