We report herein the case of a patient with Waldenström macroglobulinemia in whom an immunoblastic sarcoma produced acute jaundice and hepatic failure. Transvenous liver biopsy, performed because of coagulation deficiencies, revealed massive infiltration of the liver by the sarcoma. Immunofluorescence study showed numerous portal and sinusoidal cells stained exclusively by anti-micron anti-kappa antisera. As shown by others in the literature, this study supports evidence for the same clonal origin of the two proliferations. Chromosomal abnormalities were found in marrow and blood cells (trisomy 5 and 20, 6 q-, 7 q+, and 8 p-); their role in emergence of sarcoma is discussed. This case shows that immunoblastic sarcoma emerging during the course of Waldenström macroglobulinemia may be revealed by severe hepatic manifestations.