A clinical, enzymatic, and pathological study of an infant with the acute form of hereditary tyrosinaemia is presented. Treatment with a diet low in methionine, tyrosine, and phenylalanine was unsuccessful. A selection of specific and nonspecific hepatic enzymes, obtained at necropsy within one hour of the infant's death at 9 1/2 weeks, were studied to try to throw light on the basic defect. The major pathological findings were those of a peculiar hepatic fibrosis associated with bile retention and an abnormal grouping of hepatocytes, islet-cell hyperplasia of the pancreas, and dilatation of the proximal renal tubules. Death was precipitated by bronchopneumonia and liver failure. The difficulty in diagnosing the acute form of tyrosinaemia is pointed out, especially in differentiating it from hereditary galactosaemia (transferase deficiency) and hereditary fructosaemia. All three may present with the same clinical symptoms and liver lesions, and the distinction must be made by enzyme studies and by therapeutic trial.