The prevalence of contralateral renal abnormalities in noncystic-dysplastic kidney (NCDK) disease and renal agenesis is unknown. Twenty-four patients with NCDK disease and 16 patients with renal agenesis were studied in this 11-year retrospective study. In all patients the urinary system was evaluated by renal ultrasonography, and excretory urography or radionuclide scan. In addition, voiding cystourethrography was performed in 21 of 24 patients with NCDK disease and in 10 of 16 patients with renal agenesis. In those patients where voiding cystourethrography was performed, 14 (67%) patients with NCDK disease and 9 (90%) patients with renal agenesis had contralateral urologic abnormalities. Contralateral vesicoureteral reflux was the most common contralateral abnormality identified in 9 (43%) patients with NCDK disease and in 3 (30%) patients with renal agenesis. It is concluded that contralateral urologic abnormalities are common in patients with NCDK disease and in patients with renal agenesis and that vesicoureteral reflux is the most common contralateral abnormality noted.