Cloaca is a complex malformation in which the rectum, vagina and urinary tract open into a single common channel. Functional results after reconstructive surgery have been documented but the renal outcome is less clearly understood. The records of all patients with cloacal malformation treated at our institution from 1980 to 2000 were retrospectively reviewed to determine the renal outcome. All patients underwent serial ultrasound of the urinary tract, voiding cystography, nuclear renography, lumbosacral radiography, and measurement of serum creatinine and glomerular filtration rate when appropriate. We identified 64 patients 0.5 to 19 years old (mean age 11.2) at the time of the study. Of the 64 patients 53 (83%) were born with a structural abnormality of the urinary tract, including renal dysplasia in 17 (27%), ectopic kidney in 9 (14%), solitary kidney in 8 (13%), duplex kidneys in 6 (9%) and ureteropelvic junction obstruction in 3 (5%). Vesicoureteral reflux was noted in 34 cases (53%) and a sacral abnormality was diagnosed in 36 (57%). The glomerular filtration rate was measured in 38 patients (60%). Chronic renal failure developed in 32 patients (50%) with a glomerular filtration rate of less than 80 ml. per minute per 1.73 m. 2, including 11 (17%) who progressed to end stage renal failure, 4 who (6%) required renal transplantation and 4 who died of chronic renal failure. Of the 32 children with a glomerular filtration rate of less than 80 ml. per minute per 1.73 m. 2 44% had renal dysplasia, 25% had a solitary kidney, 44% had sacral anomalies, 72% had vesicoureteral reflux and 47% had secondary renal scarring. Renal impairment causes significant morbidity and mortality in patients with a cloacal malformation. These patients need a complete nephrourological assessment in the neonatal period due to the high incidence of urinary tract and sacral anomalies as well as careful post-reconstruction followup.