We examined β2-microglobulin (B2MG) excretion, an index of tubular function, in patients with hepatorenal syndrome, in whom tubular function is generally regarded as normal. Urine B2MG was significantly higher in these patients than in control patients with normal serum creatinine concentration. Patients with high urine B2MG concentration had markedly higher serum bilirubin than did patients with normal values (31 ± 3 vs. 10 ± 8 mg%, p < 0.001), whereas prothrombin activity, serum albumin and serum B2MG concentration were similar. A “threshold” serum bilirubin concentration of about 23 mg% differentiated patients with normal and high urine B2MG values. Renal morphology at autopsy was unremarkable in both groups. Tubular dysfunction, manifested by increased urinary excretion of B2MG, occurs in patients with hepatorenal syndrome and deep jaundice. This measurement cannot, therefore, be used to make a diagnosis of acute tubular injury, as due to aminoglycosides, in such patients.