Several studies have investigated leptin levels in patients with multiple sclerosis (MS) with somewhat conflicting results. They have all focused on patients with established relapsing-remitting (RR) MS but have not specifically looked at patients with clinically isolated syndrome (CIS) suggestive of MS, in the early stages of disease. In this study, serum leptin levels were measured in 89 treatment-naïve patients with CIS (53 patients) or RRMS (36 patients) and 73 controls searching for differences between the groups and for associations with several disease parameters. The expected significant sexual dimorphism in leptin levels (higher levels in females) was observed in both MS patients and controls. Increased leptin levels were found in female patients with RRMS compared to female controls (P = .003) and female CIS patients (P = .001). Female CIS patients had comparable levels to controls. Leptin levels correlated positively to disease duration, but not to EDSS, in female patients with RRMS. The results of the present study do not indicate involvement of leptin in the early stages of MS. Normal leptin levels in patients with CIS suggest that leptin does not have a pathogenic role. The ratio leptin/BMI increases during disease course in female MS patients in a time-dependent and disability-independent manner.