Recently, it has been noted that radical molecules participate in the pathogenesis of some of the complications associated with long-term dialysis therapy, such as amyloidosis. Since allantoin is produced from uric acid almost exclusively by the hydroxy radical, we investigated the plasma concentrations of allantoin in 71 patients with chronic renal failure who were on maintenance hemodialysis. Our specific objective was to investigate the relation of allantoin to the plasma concentrations of beta2-microglobulin and methylguanidine to evaluate the potential of allantoin as a significant parameter of the radical reaction. Although plasma allantoin was not detected in 15 healthy controls, plasma concentrations of allantoin increased markedly in all of the hemodialysis patients (42.6 +/- 37.7 nmol/mL) and wide variation was observed from 4.3 to 185.2 nmol/mL. In addition, we confirmed significant correlations between plasma concentrations of allantoin and beta2-microglobulin as well as serum concentrations of methylguanidine/creatinine and hyaluronic acid (r - 0.456, p < 0.0005, r = 0.313, p < 0.005, r = 0.368, p < 0.01, respectively). Although evidence for a direct link between plasma allantoin levels and amyloidosis was not obtained, the increase in the plasma concentrations of allantoin suggested its clinical significance as a parameter of the radical reaction that is thought to participate closely in the pathogenesis of dialysis-related amyloidosis. From a practical point of view, the measurement of plasma allantoin can be expected to become a significant method of measuring radical members which may be useful in the prophylaxis of some of the complications induced by free radicals.