This study aimed at investigating leptin levels in male diabetes type I patients who were on insulin treatment and also healthy sedentary males. The study included 10 male type I diabetes patients and 17 healthy sedentary males. Leptin levels of type I diabetes patients and healthy sedentary males with body mass index (BMI) over 25 kg/m2 were evaluated separately. The relation between serum leptin, max VO2, blood lactic acid levels before and after exercise, and effort durations of participants were investigated. At the end of the tests, no difference was found between leptin levels, max VO2 values, lactic acid values before exercise, and test durations of male type I diabetes patients and healthy sedentary males (p > .05), whereas lactic acid levels after exercise were found to be lower in healthy sedentary males (p < .05). Leptin levels in the group with BMI above 25 kg/m2 were higher than those in the group with BMI below 25 kg/m2 (p < .001). It was also seen that max VO2 values and test durations were higher in the group with BMI below 25 kg/m2 (p < .05). In conclusion, leptin levels of male type I diabetes patients are close to those of healthy sedentary males. The increase in leptin levels in both groups is in proportion to the BMI of individuals.