This study was performed to clarify serum beta-2 microglobulin (beta 2-M) level and its change in 50 CAPD and 56 HD patients. There was significant correlation between duration of dialysis and serum beta 2-M level in CAPD and HD patients treated under 12 months, but no correlation in those treated over 12 months. Serum beta 2-M level was 33.5 +/- 9.1 mg/l in 45 CAPD patients treated over 12 months, and 46.2 +/- 21.1 mg/l in 35 HD patients. In 26 CAPD patients treated over 12 months, clearance and removal of beta 2-M were 1.0 +/- 0.3 ml/min and 43.0 +/- 17.8 mg/day. There was significant correlation between dwell time and beta 2-M removal (p less than 0.01), and these results suggested beta 2-M was removed by diffusion. Because CAPD treatment can lower serum beta 2-M level compared to HD, there is possibility that CAPD is useful at prevention of dialysis associated amyloidosis.