A 58-year-old female was admitted to our hospital because of anemia in March 1987. Monoclonal protein (IgA, kappa) was detected and a diagnosis of multiple myeloma was made. Partial remission was obtained after VAD therapy with alpha-interferon. In December 1989, she was readmitted because of a pathological fracture of the left humerus. A white blood cell count was 4400/microliters with 30% myeloma cells and the urine protein (Bence Jones protein) was 26 g/day. Systemic chemotherapy was not effective. She developed pleural and pericardial effusions, bone mass, disturbance of consciousness and died of respiratory failure only 3 months after readmission. The pleural and pericardial fluids contained many myeloma cells. c-myc gene rearrangement was detected in myeloma cells obtained from the pleural fluid using c-myc exon1 and exon2 probes. The levels of interleukin-6 (IL-6) measured by ELISA was 107.4 pg/ml in serum, 56.2 pg/ml in pleural fluid and 780.0 pg/ml in pericardial fluid. Because of the lack of any overt infectious focus, the level of IL-6 appears to have been related to aggressive proliferation of myeloma cells. It was of interest that C-reactive protein, induced by IL-6, was a good marker reflecting disease activity.