Hemodialysis is not an absolute prerequisite for the formation of beta 2-microglobulin amyloidosis, but it enhances the progression of this complication related to long-standing renal failure. Thus the clearance and turnover of beta 2-microglobulin seems to play a major role in this disease. In a prospective multicenter study the beta 2-microglobulin clearance was studied in 87 patients starting hemodialysis. Serum samples were taken prior to and after the first dialysis session and also before and after dialysis at 4, 6, 12, 16, 26 and 52 weeks. Patients were either treated by cuprophane or a polyacrylonitril membrane. At the start, the mean serum beta 2-microglobulin level was about 18 mg/L in patients treated with a cuprophane membrane, but the levels increased after hemodialysis and reached a plateau, which was always higher than in those treated with polyacrylonitril, which cleared beta 2-microglobulin from the serum. However, after 12 months the difference was no longer significant. Thus beta 2-microglobulin excretion during dialysis differs between the two membranes, but seems to lose its significance for the beta 2-microglobulin serum level in chronic hemodialysis treatment.