From 1972 to 1974 a collaborative prospective study was undertaken to determine the incidence, causes and clinical course of chronic renal failure (CRF) in pediatric patients (age 1/2-16 years) in Switzerland. In addition, information concerning the occurrence of terminal CRF observed between 1967 and 1971 was obtained. CRF is defined as persistently elevated serum creatinine greater than 2 mg/100 ml. The small size of Switzerland (population 6.27 million) and the availability of specialized medical care favoured relative completeness of data. a) Non-terminal CRF. - Prevalence was 4.5 per million total population = 18.5 per million children (1/2-16 years old) (n = 28 at 31. 12. 1974). Causes: renal disease was acquired in 14% and congenital in 86% (hereditary nephropathies excluding polycystic kidneys 4%, polycystic kidneys 14%, hypoplasia 14%, urinary tract abnormalities 54%). Progression of CRF: mean time interval in which serum creatinine doubled was, in the group of acquired renal diseases, 8.3 months (n = 6); in hereditary nephropathies 14.3 months (n = 4); in the group of polycystic kidneys and hypoplasia 41.2 months (n = 6), and in patients with urinary tract abnormalities 35.8 months (n = 15). Body height was reduced in all groups as compared with normal children: 1.0 SDS (standard deviation scores) in acquired renal diseases (n = 9), 5.1 SDS in cystinosis (n = 2), 0.4 SDS in polycystic kidneys (n = 4), 1.6 SDS in hypoplasia (n = 5), and 2.3 SDS in urinary tract abnormalities (n = 16). b) Terminal CRF. - Mean yearly incidence (1967-1974, n = 68) was 1.36 per million total population = 5.6 per million children (1/2-16 years). Causes: In contrast to the situation in non-terminal CRF, acquired diseases were observed in 56% of all patients. Hereditary nephropathies were found in 12%, polycystic kidneys in 1.5%, hypoplasia in 4.5%, and urinary tract abnormalities in 26%. Between 1970 and 1972 , 7 of 21 patients, and during 1973/1974 13 of 16 pediatric patients, were treated by intermittent hemodialysis and renal transplantation. On 31. 12. 1974, 18 of these 20 treated patients were alive, and of these 14 had a functioning graft.