Background: Although there is a clear relationship between body mass index and leptin levels, few authors have addressed the possible influence of ethnic factors on these levels. Aim: To measure serum leptin in three different Chilean aboriginal populations. Subjects and methods: Fasting serum leptin and insulin levels were measured by radioimmunoassay in 345 rural mapuche individuals, 247 rural aymara subjects and 162 urban mapuche subjects. A body mass index of 27.5 kg/m2 was used as cutoff point to classify study subjects. Results: Among the three ethnic groups, women had serum leptin levels three times higher than men. In all three ethnic groups, there was a significant association between leptin levels, body mass index and gender (r2= 0.32 and 0.5 p <0.001, in rural mapuche, r2= 0.32 and 0.5 p <0.001, in aymara and r2= 0.24 and 0.49, p <0.001 in urban mapuche populations). No differences in leptin levels were observed for the interaction between age and insulin. The increments per quartile in leptin levels were lower among mapuche than aymara individuals. Conclusions: Rural mapuche individuals have a high frequency of obesity. However their leptin levels are lower than those of aymara or urban mapuche populations. The higher leptin levels observed in urban mapuche subjects could be due to environmental influences. (Rev Méd Chile 2000; 128: 45-52)