Liver biopsy is the recognized gold standard for liver fibrosis staging. The aspartate aminotransferase to platelet ratio index (APRI) has been proposed as a noninvasive and readily available tool for the assessment of liver fibrosis in chronic hepatitis C (CHC). This study aimed to validate, in a Mexican tertiary health care setting, the diagnostic usefulness of APRI in CHC, nonalcoholic fatty liver disease (NAFLD) and autoimmune hepatitis (AIH). In an observational, cross-sectional, comparative and retrolective fashion, consecutive patients with CHC, NAFLD or AIH were evaluated. Fibrosis was staged using the METAVIR scale. Receiver operating characteristic ROC curves were constructed for significant fibrosis, advanced fibrosis and cirrhosis. One-hundred-sixty-four CHC, 30 NAFLD and 42 AIH patients were evaluated. For the diagnosis of significant fibrosis, APRI values delimited an area under de ROC curve (AUC) of 0.776 in CHC, 0.564 in NAFLD, and 0.602 in AIH patients. For advanced fibrosis, the AUCs were 0.803, 0.568 and 0.532 in CHC, NAFLD and AIH patients, respectively. For cirrhosis, AUCs were 0.830 and 0.599 in CHC and AIH patients. In conclusion, APRI can be a useful noninvasive alternative for the diagnosis of significant fibrosis and cirrhosis in our CHC patients. APRI values of < or = 0.3 and < or = 0.5 rule out significant fibrosis and cirrhosis, and a value of > or = 1.5 rules in significant fibrosis. In patients with NAFLD, APRI values tend to increase with the degree of fibrosis, suggesting that it could be useful in this disease. APRI appears to be of no value in patients with AIH.