Two patients with multiple myeloma who appeared to be producing ammonia are reported. Both patients showed hyperammonemia and amino acid disturbances, such as a low Fischer ratio. One patient had Bence Jones protein (λ) type myeloma and became comatose, but the hyperammonemia and disturbance of consciousness were improved by chemotherapy for the myeloma. The other patient had IgA gκ type myeloma and somnolence and died of malignant pleurisy despite intensive chemotherapy. Autopsy showed widespread multiple myeloma and an almost normal liver. Ammonia levels in the supernatant of cultured myeloma cells from the patient’s pleural effusion increased almost linearly from the time of cell seeding. These observations showed that ammonia was produced at a high level by these human myeloma cells. We also found that one of the common myeloma cell lines, RPMI 8226, could produce ammonia as well.