The deterioration rate of creatinine clearance (CCr) was studied in 40 children with chronic renal failure (CRF) on conservative treatment followed up for at least 1 year (range 1-12). The deterioration rate of CCr was significantly (p less than 0.01) higher in glomerulopathies (G) than in hypoplasias (H) and in vascular nephropathies (VN) and significantly (p less than 0.01) higher in hereditary nephropathies (HN) than in VN. The differences in the deterioration rate of CCr between H and HN and between H and VN were not explainable on the basis of the different age at diagnosis or of the different prevalence of hypertension. These data indicate that the primary renal disease is important in determining the progression of CRF.