Current studies have reported that free radical molecules had anticipated intimately with the pathogenesis of some complications specific for a long term hemodialysis. Among substances generated by free radical reaction, methylguanidine (MG) had been well known as most popular parameter in hemodialyzed patient, but allantoin (Alt) had been reported to be generated from uric acid by hydroxyradical, too. In this study, we measured serum level of Alt in long term hemodialyzed patient and analysed in the comparison with serum level of sialic acid, MG, and MG/Cr value. Serum level of Alt in patients group varied from 2.6 to 105.9 nmol/ml, which was significantly higher as compared with exclusively undetectable level in normal volunteers (n = 15). Serum level of Alt in each patient showed a significant correlation with serum level of a sialic acid (r = 0.400 p < 0.02). Furthermore, although serum level of Alt failed to show a significant correlation with serum level of MG, but showed a significant correlation with MG/Cr value. (r = 0.485 p < 0.01). In addition, higher level could be found in patient group having higher serum level of MG or MG/Cr value. Based upon those results, we proposed that a measurement of serum level of Alt could be of value in detection of free radical reaction in long term hemodialyzed patient.