From October 1987 to November 1993 we evaluated the serum levels of ammonia and amino acids in 85 patients with multiple myeloma. Six of the 85 cases of multiple myeloma demonstrated hyperammonemia and none of the known causes of hyperammonemia, such as liver failure, could be identified in these patients. All six patients also showed serum amino acid disturbances and conscious disorders in various degrees. In this study we compared these abnormalities in multiple myeloma with those in chronic liver failure (n = 14), the basic diseases of which were liver cirrhosis in six cases and liver cirrhosis complicated hepatocellular carcinoma in eight cases. There was a marked difference in the levels of individual serum amino acids between these two groups. The level of glycine was significantly higher in the multiple myeloma group (P < 0.001); on the other hand, that of tyrosine was significantly higher in the liver failure group (P < 0.005). The histidine (P < 0.005) and arginine (P < 0.005) levels were lower in the myeloma group. The ratio of glycine to tyrosine (Gly/Tyr) was 16.7 +/- 4.85 in the myeloma group and 1.7 +/- 0.12 in the liver failure group. The ratio of glycine to tyrosine was an important criterion for differential diagnosis.