PURPOSE: The urinary mass screening program for the detection of proteinuria had been performed for a school age population in the city of Seoul since 1981. However, systematic evaluation for this program had never been done. We, therefore, designed a systematic evaluation in corporation with the Seoul School Health Center for students with proteinuria identified in the mass screening to determine the prevalence of asymptomatic proteinuria, and to estimate the risk of incipient renal diseases and incidences of some of them. And also, we attempted to reveal the significance of urinary mass screening. METHODS: In the period of 8 years betwen 1987 and 1994, annually about a half million students, who comprised 3 different age groups, 5th grade of elementary school, 2nd grade of junior school and 2nd grade of high school were chosen, corresponding to the approximate ages of 11, 14 and 17 years, respectively. These subjects accounted for 25% of total school children in Seoul. The screening program was carried out in 3 steps. The 1st test was performed at schools and the 2nd at the School Health Center. Those students who showed proteinuria in the 1st and 2nd tests were referred to the hospital. Laboratory examinations including renal biopsies were performed in those students with pathologic proteinuria to clarify the incipient renal diseases. RESULTS: 1) The prevalences of asymptomatic proteinuria were 0.2-0.36% (average 0.28%) in the 1st test. It peaked at the age of 14 years 0.36%, compared to 0.2% in the 11 age group and 0.25% in the 17 age group. 2) There was a difference of asymptomatic proteinuria by sex and age, higher prevalence in the female in the 11 age group but higher in the male in the 17 age group. 3) The distribution of proteinuria in accordance with various types classified by us at the 3rd test showed as follows: transient proteinuria (type 1) 17%, orthostatic proteinuria (type 2) 55%, constant proteinuria (type 3) 6%, proteinuria with hematuria (type 4) 20%, isolated proteinuria (type 5) 2%. Pathologic proteinuria comprised of type 3, 4 and 5, accounted for 28%, which was 1/10 of proteinuria in the 1st test. 4) The annual incidence of renal glomerular diseases among the age goup of 7-18 years was estimated to be 2.8 per 10,000. 5) Renal pathologic studies on 80 children with proteinuria and hematuria (66) or constant proteinuria (14) revealed IgA Nephropathy (38.9%) was the most frequent glomerular diease. MPGN 10.0%, MGN 7.5%, Reflux Nephropathy 7.5%, FSGS 6.2% were followed, respectively. HBV asssociated Nephropathy was 7.5%. 6) 4 of Reflux Nephropathy and 2 of FSGS were found in the 14 children with constant proteinuria in the screening test. Among these children, 5 progressed to chronic renal failure. 7) The annual incidence of IgA Nephropathy among the age group of 7-18 years was 1.1 per 10,000. 8) The morbidity of chronic renal failure was estimated to be 9.8 per 1 million of 7 to 18 years age group. CONCLUSIONS: 1) Most Students, approximately 90%, who were found to have asymptomatic proteinuria in the initial screening were normal so clinical evaluations for them should be very much cautious. Only 10% of them had pathologic proteinuria which denoted incipient renal diseases. 2) Annual incidences of incipient renal glomerular diseases, IgA Nephropathy and chronic renal failure were estimated to be 2.8 per 10,000, 1.1 per 10,000 and 9.8 per 1 million among school children in Seoul. 3) Because of the high incidence of significant glomerular changes and the high rate of progression to chronic renal failure in children with constant proteinuria, it should be regarded as serious one. 4) The definite conclusion that a urinary mass screening program can alter the prognosis of children with renal diseases identified in the mass screening can not be drawn with this study. Further study must be necessary.