Leptin is a protein hormone controlling food intake and energy expenditure. In all infections including parasitic infections there is loss of appetite and anorexia. The aim of the present study was to clarify the relationship between intestinal parasites and serum leptin concentrations in children and adults. Forty patients with intestinal parasites and 34 healthy subjects took part in this study. Body weight, height and body mass index (BMI) were measured for all patients and controls. Patients were grouped according to age and parasitic infections (Giardia intestinalis, Blastocystis hominis, Enterobius vermicularis, Entamoeba histolytica/Entamoeba dispar, Entamoeba coli). Serum leptin concentrations were detected by immunoenzymometric assay using the Biosource Leptin EASIA kit. Statistical analysis was made by Mann-Whitney-U test using SPSS version 10.0. In children, the serum leptin levels were not statistically significant (patient: 1.49+/-1.97 ng/ml, control: 3.48+/-4.97; p=0.854) But for adults, although the BMI of patients were similar to that of the control group; the leptin levels of patients were low. These levels were not significant (patients: 9.06+/-10.34; controls: 4.7+/-9.02 ng/ml; p=0.271). There was no statistical difference for leptin levels in patient groups, children and adults due to intestinal parasitic infections. Further investigations are needed.