Establishing a diagnosis of fulminant Wilson's disease can be difficult because Kayser-Fleischer rings may not be present and parameters of copper metabolism, including serum and urinary copper, and serum ceruloplasmin levels are neither specific nor diagnostic. In this study, ratios of both the serum alkaline phosphatase to total bilirubin and aspartate transaminase to alanine transaminase were constructed to evaluate their usefulness in differentiating fulminant hepatic failure caused by Wilson's disease (n = 6) from other etiologies (n = 43). An analysis of the data showed that cutoff values of less than 2.0 for the alkaline phosphatase-total bilirubin ratio and greater than 4.0 for the aspartate transaminase ratio were associated with a diagnosis of fulminant hepatic failure caused by Wilson's disease only (P less than 0.001). The alkaline phosphatase-total bilirubin ratio of less than 2.0 provided 100% sensitivity and specificity in identifying fulminant hepatic failure caused by Wilson's disease from other types of fulminant hepatic failure.