Beta 2-microglobulin (beta 2M) amyloidosis is common in patients on long-term hemodialysis, but the clinical conditions associated with disease activity are poorly understood. This study was designed to determine if the serum amyloid P (AP) component concentration is predictive of beta 2M amyloid disease activity. Serum AP component concentrations were determined by rocket immunoelectrophoresis and beta 2M concentrations by a commercially available kit. Radiographic evidence of beta 2M amyloidosis was determined from bone films of the hips, shoulders, and hands. Serum AP component concentrations were not different in dialysis and control patients. However, AP component concentrations in long-term (greater than or equal to 5 years) dialysis patients were significantly lower than in short-term (less than 5 years) dialysis patients (43.0 +/- 16.9 micrograms/mL [n = 28] v 56.0 +/- 18.3 micrograms/mL [n = 31], P less than 0.05). The patients on hemodialysis for 5 or more years who had radiographic evidence of severe beta 2 M amyloidosis were significantly older (57.9 +/- 9.5 v 38.3 +/- 11.3 years, P less than 0.001) and their serum AP concentrations were significantly lower (34.3 +/- 15.0 v 50.1 +/- 15.6 micrograms/mL, P less than 0.05) than long-term dialysis patients without radiographic evidence of disease. Stepwise regression analysis showed that the patient's age and serum AP component concentration were predictors of radiographic evidence of beta 2 M amyloidosis. Thus, serum AP component concentrations are decreased in long-term dialysis patients, suggesting accelerated deposition into amyloid deposits.(ABSTRACT TRUNCATED AT 250 WORDS)