OBJECTIVES To analyze the data of our series of patients either operated in the first year of life or managed conservatively because of prenatally detected ureteropelvic junction obstruction, with particular consideration for the significance of renal pelvis diameter. METHODS A total of 56 patients with prenatal ultrasonographic findings suggestive of ureteropelvic junction obstruction were enrolled in this study. Operated group and control group consisted of 35 and 21 patients, respectively. We compared postnatal imaging findings of patients managed either conservatively or operatively. The diagnostic accuracy of imaging studies in confirming the diagnosis of obstruction and determining the need for surgery was analyzed. RESULTS If relative function of the involved side being <40% was accepted as the gold standard, renal pelvis diameter was found to be a statistically significant predictor of surgical intervention based on the results of receiver operating characteristic curve analysis. The critical diameter was found to be 22 mm, at which sensitivity and specificity of identifying surgical candidates were calculated to be 78.9% and 50%, respectively. In addition, if we consider that having undergone a pyeloplasty procedure is the most accurate definition of ureteropelvic junction obstruction, the critical pelvis diameter was found to be 19 mm. Renal pelvic dilation >19 mm defines obstruction that may necessitate surgical treatment with a sensitivity of 93.9% and specificity of 71.4%. CONCLUSIONS Renal pelvis diameter may indicate the need for surgical intervention or further assessment to define clinical significance more accurately.