A retrospective study was carried out on patients with gastrointestinal complaints, who were accidentally found to have varying degrees of hydronephrosis between October 1989 and April 1991. The criteria for hydronephrosis given by Ellenbogen et al were used for ultrasonographic diagnosis. Grade 0 and 1 were classified as mild degree and grade 2 and 3 as moderate and severe, respectively. The causes of hydronephrosis were determined by intravenous pyelography, retrograde pyelography, and/or computed tomography. Of 44 patients with hydronephrosis who had complete studies, 20 had a mild degree of hydronephrosis, 18 had a moderate degree, and 6 had a severe degree. All 24 patients with moderate and severe degrees of hydronephrosis had obstruction as the etiology, but only 50% of the cases with mild hydronephrosis were found to have pathological lesions. Urolithiasis (79.4%) was the most common cause of hydronephrosis in this study. Benign prostate hypertrophy, carcinomas, external compression, and tuberculosis followed. Hydronephrosis found accidentally by sonography in the patients with gastrointestinal complaints is significant.