The ratio of serum aspartate aminotransferase to alanine aminotransferase (AST/ALT ratio) has been proposed as a noninvasive method of assessing liver fibrosis and cirrhosis. Our aims were to confirm the usefulness of the AST/ALT ratio in diagnosing cirrhosis noninvasively as well as to verify the existence of a relationship between the ratio and liver functional impairment. In all, 348 patients (177 with chronic hepatitis, 171 with cirrhosis) were retrospectively evaluated and the AST/ALT ratio was related to monoethyl glycine xylidide (MEGX) formation. Moreover, in a subgroup of 54 patients we analyzed the relationships among the AST/ALT ratio and indocyanine green clearance and half-life. The AST/ALT ratio was able to separate patients with mild fibrosis from those with severe fibrosis and cirrhosis. The AST/ALT ratio, MEGX, prothrombin activity, and platelet count were selected by multivariate analysis as variables associated with cirrhosis. The AST/ALT ratio showed significant correlations both with MEGX formation and with indocyanine green clearance and half-life. The alterations of indocyanine green kinetics, which depend upon liver blood flow and uptake, were likely due to progressive fibrosis. These findings might partially explain the increase in the AST/ALT ratio as disease progresses.