PURPOSE: Considering that renal biopsy is not routinely indicated in nephrotic syndrome in children and the risk of the procedure, we studied that it is possible to predict steroid responsiveness in nephrotic syndrome and the difference in responsivenss is related with the histopathologic type using urinary beta2-microglobulin and N-Acetyl-beta-D-Glucosaminidase activity as a safe and noninvasive method. METHODS: We measured serum creatinine, albumin, cholesterol and urinary creatinine, protein, B2M, NAG at the time of admission and after 2 weeks of treatment in 37 cases of primary nephrotic syndrome in children who were admitted to the Pediatric department of Chung-Ang Medical Center between March 1, 1994 and June 30, 1996. RESULTS: The results were as follows : 1) When urinary B2M value of less than 1000microgram/g creatinine was used as the cut-off value, the test identified steroid responsiveness with a sensitivity of 100%, a specificity of 80%. When urinary NAG activity of less than 100U/hr/mg creatinine was used as the cut-off value, the test identified steroid responsiveness with a sensitivity of 100%, a specificity of 81.8%. 2) Pretreatment urinary B2M levels in Good-response and Poor-response group were 559.8+/-718.5microgram/g creatinine, 3599.1+/-4956.7microgram/g creatinine, respectively. There was statistical significance between the two groups (P<0.05). 3) Pretreatment urinary NAG activity in Good-response and Poor-response group were 42.6+/-23.6U/hr/mg creatinine, 79.6+/-80.1U/hr/mg creatinine, respectively. There was statistical significance between the two groups (P<0.05). CONCLUSIONS: In conclusion, measurement of urinary B2M level and NAG activity for early detection of renal tubular damage is helpful in prediction of steroid responsiveness of primary nephrotic syndrome. If the urinary B2M level and NAG activity are elevated, the patient is suspected to steroid resistance. Therefore, prolonged steroid therapy should be avoided in such patients because of their greater chance of not responding. This approach would reduce the many serious side effects of prolonged treatment in patients who are unlikely to benefit.