Nowadays, in spite of the remarkable development of the results of open heart surgery, the incidence of postoperative acute renal failure [ARF] has been increased due to the expansion of the candidates and prolonged operation time for complicated cases. It is also well known that ARF after open heart surgery, if once occurred, is a critical complication, therefore early diagnosis and treatment about it are very important for prognosis. Recently a low molecular weight [2 * microglobulin [[2* MG]has been used as a indicator of renal function. Because of the properties of [2 * MG, serum concentration of it is increased in glomerular dysfunction and urine excretion of it is increased in tubular dysfunction. Author studied about the perioperative changes of serum [2* MG and BUN concentration in 25 children and 30 adults for evaluation of significances of [2* MG as a parameter of postoperative renal function in open heart surgery, and the results were obtained as follows. l. In open heart surgery, the serum [2* MG concentration was elevated postoperatively in the all cases from the first postoperative day. 2. There were a significant correlation between the preoperative BUN and [2* MG concentration [P< 0.01]. The correlation factor[r] in child group was 0.8512, and in adults 0.8636. 3. The maximum level of serum [2* MG in the both child and adult groups were noticed in 4th postoperative day as 2.61*0.80mg/ 1 in child group and 3.39*1.47 mg/ 4 in adult group, and there was a significant difference between the two groups statistically[P < 0.05]. 4. The pattern of changes of serum concentration of [2* MG with time was very similar with the changes of BUN, but in a case of ARF [expired with it] the changes of [2* MG was more remarkable. 5. There was a significant differences in the maximum level of [2* MG between the 2 group according to the ECC time [groups of below and above 60 minutes] [P< 0.01].