['To investigate the relationship between the histopathologic findings and the postoperative course of children surgically treated for ureteropelvic junction (UPJ) obstruction.', 'Twenty-eight patients operated for unilateral UPJ obstruction from 1998 to 2005 with adequate histopathologic specimens and postoperative follow up were retrospectively reviewed. Specimens were stained using elastic van Geisson to differentiate smooth muscle from collagen and elastin. Postoperative follow up included renal ultrasound (U/S) and diuretic renogram studies.', 'Twelve patients with mean renal pelvis smooth muscle thickness (mRPSMT) of 136.97 ± 34.17 improved on the 6(th) postoperative month. Nine patients that improved after 9 months postoperatively had mRPSMT=173.61 ± 33.91. The rest 7 patients that improved on the 12(th) postoperative month had mRPSMT=258.78 ± 96.09. Correlation between renal pelvis smooth muscle and time of postoperative improvement was extremely significant (r = 0.7928, p < 0.0001).'] Does [ Thickness of the renal pelvis smooth muscle indicate the postoperative course of ureteropelvic junction obstruction treatment ]?