Ninety-six children, aged 4 to 14, with congenital obstructive uropathies were subjected to 131I-hippuran renography, followed by mathematical processing of renographic curves, the measurement of blood mean molecular levels, and blood and urinary immunochemical tests before, and 1 month as well as 1 year after surgery. Intraoperative renal biopsy was taken from 20 patients with unilateral hydronephrosis. Latent chronic renal failure (CRF) was identified where a deficiency of total renal clearance of 131I-hippuran (20 to 56%) was combined with a rise in blood mean molecules from 0.3 to 0.41 conventional units at 254 nm. Latent CRF was detected in 40 of 64 children with unilateral uropathy and in all 32 patients with bilateral uropathy. In patients with unilateral hydronephrosis, the presence of CRF was unrelated to the morphological pattern of pyelonephritis in the affected kidney. Children with latent CRF showed high levels of urinary IgG and albumin and blood mean molecules. One year after the operation, renal function improved in patients whose contralateral kidney had no secretory deficiency. One year after surgery, renal reabsorption mechanisms tended to recover in CRF-free patients only. The clinical pattern of latent CRF and its elimination following surgery were unrelated to roentgenologic markedness of hydronephrosis and VUR, but were dependent on the recovery of compensatory mechanisms in the contralateral kidney. By the end of the first postoperative year, latent CRF was diagnosed in 31% of children with unilateral hydronephrosis (as compared to the preoperative 68%), 43% (vs. the preoperative 60%) of children with unilateral VUR, and in 44% (vs. the preoperative 50%) of patients with unilateral neuromuscular ureteral dysplasia.(ABSTRACT TRUNCATED AT 250 WORDS)