We assessed activity energy expenditure (AEE) in Mexican-American (MA) and European-American (EA) children. Total energy expenditure (TEE) was measured using the doubly-labeled water method; AEE was calculated as the difference between TEE and resting EE (REE), and physical activity level (PAL) was calculated as TEE/REE. Groups were comparable for age, sex and body mass index (BMI). REE did not differ between groups. The boys did not differ in TEE, AEE, or PAL (MA vs. EA, respectively: TEE, 7.9+/-1.5 vs. 7.5+/-0.9 MJ x d(-1); AEE: 64.9+/-24.7 vs. 65.3+/-22.3 kJ x kg(-1) x d(-1); PAL: 1.57+/-0.18 vs. 1.58+/-0.19 kJ x kg(-1) x d(-1)). MA girls had lower TEE, AEE, and PAL than EA girls (TEE: 6.8+/-0.9 vs. 8.1+/-0.8 MJ x d(-1); AEE, 37.3+/-15.9 vs. 64.9+/-24.7 kJ x kg(-1) x d(-1); PAL, 1.40+/-0.12 vs. 1.57+/-0.18; P <0.005). Results suggest that these MA girls were expending less energy than EA children of comparable body size due to a reduced activity energy expenditure.