The upper limit of normal for ALT activity has been recommended to be lowered to < or = 30 U/L in men and < or = 19 U/L in women. These changes have been suggested to be diagnostically useful in subjects with nonalcoholic fatty liver disease (NAFLD). Our aim was to investigate the prevalence and spectrum of NAFLD with regard to the new ALT guidelines in 233 women with class II/III obesity. We compared our prior reference range for ALT (ULN < or = 30 U/L in women) with the new standard. Our study demonstrates that only 86 patients (36.9%) would be classified as having normal ALT levels compared with 169 patients (72.5%) by the new and old standards, respectively. In patients with normal ALT activity (new vs. old standard), the prevalence of fatty liver (FL: 39.5% vs 40.2%), portal fibrosis, and steatosis (IPF: 37.2% vs. 33.7%) and nonalcoholic steatohepatitis (NASH: 23.3% vs. 26%) were similar. In comparison, newly defined patients with elevated ALT levels (>19 U/L) demonstrated an increased prevalence of FL (36%) and IPF (11.6%) but a 23.8% decrease in the prevalence of NASH as compared with the old standard. The sensitivity and specificity for NASH were 42% and 80% (ALT > 30 U/L) compared with 74% and 42% (ALT > 19 U/L). In conclusion, a significant increase in the prevalence of FL and IPF is detected in subjects with elevated ALT levels with the application of the new standard. However, the diagnostic utility for ALT to identify NASH or IPF remains poor, and significant healthcare expenditures may be incurred if this standard is adopted.