The aim of this study was to compare the serum complement C3 concentration between non-obese young insulin-sensitive and insulin-resistant Mexicans. A cross-sectional study was carried out in 28 healthy, non-obese [Body mass index (BMI) < 26 kg/m2] young (age 19-25 yr), male volunteers to measure the serum C3 concentration. In accordance with the constant for rate serum glucose disappearance (KITT) obtained from the insulin tolerance test, the subjects were divided into quartiles, considering as insulin-resistant individuals those in quartile 1, and insulin-sensitive subjects those in quartile 4. Serum C3c concentration was measured by a nephelometric method. Other biochemical characteristics were measured, like lipid profile and uric acid using enzymatic techniques. The serum C3 concentrations were similar (p = 0.949) between insulin-resistant and insulin-sensitive groups respectively (1.4 +/- 0.2 vs 1.3 +/- 0.1 g/l). There were no significant correlations between serum C3 concentrations and serum glucose, insulin levels, KITT and lipid profile. In conclusion, the serum complement C3c concentrations were similar between non-obese young insulin-sensitive and insulin-resistant Mexicans.