The aim of our study was to examine the changes in hypertransaminasemia after weight reduction in obese patients with and without nonalcoholic fatty liver disease (NAFLD) and the relation with insulin resistance. A population of 142 obesity nondiabetic was treated with a hypocaloric diet (1520 kcal) during 3 months. Patients were classified as group I (control, n=112) when serum (alanine aminotransferase) ALT activity was normal or group II (NAFLD, n=30) when serum ALT activity was greater than the upper limit of normal reference laboratory (>or=43 UI/L). In control group, body mass index (BMI) (35.3+/-5.6 vs. 33.5+/-5.5: p<0.05), weight (89.6+/-17.2 kg vs. 85.6+/-16.5 kg: p<0.05) and insulin (15+/-7.9 mUI/L vs. 13.1+/-7.4 mUI/L: p<0.05) levels decreased. In NAFLD group, BMI (37.1+/-4.2 vs. 35.1+/-4.6: p<0.05), weight (101+/-22 kg vs. 96.4+/-11.6 kg: p<0.05), insulin (26.8+/-8.9 mUI/L vs. 12.7+/-8.4 mUI/L: p<0.05) and HOMA (3.1+/-1.6 vs. 1.9+/-1.7: p<0.05) decreased. Liver function improved in both groups (ALT; group I 19.9+/-4.6 UI/L vs. 18.3+/-3.9 UI/L: p<0.05 and group II 37.8+/-4.2 UI/L vs. 27.1+/-7.8 UI/L: p<0.05), (aspartate aminotransferase activity, AST) group I 22.1+/-8.2 UI/L vs. 19.9+/-7.1 UI/L: p<0.05 and group II 73.3+/-11.3 UI/L vs. 38.1+/-11.6UI/L: p<0.05). We showed that weight reduction secondary to a hypocaloric diet were associated with improvement in hypertransaminasemia and insulin resistance in NAFLD patients.