To assess the sonographic frequency of simple renal cysts in children, the authors retrospectively reviewed the results of abdominal sonographic studies of 16,102 children performed over a 5-year period between January 1, 1985, and December 31, 1989. Patients with abnormal renal function, dysplastic kidneys, or a family history of polycystic kidney disease were excluded from the study. The authors' review of the sonograms revealed 37 simple cysts in 35 patients (0.22%); the cysts were evenly distributed by age and sex and measured from 0.3 to 7.0 cm in maximum diameter. Sixteen cysts (43%) were in the upper pole of the right kidney. Follow-up sonographic studies of 23 cysts in 22 patients for up to 5 years showed no change in size in 17 cysts (74%). The largest cyst was drained percutaneously; all other cysts were managed conservatively. No patient showed deterioration of renal function. Therefore, the authors concluded that in a pediatric patient demonstrating normal renal function, no further intervention is necessary when a simple renal cyst is identified at sonography.