A morphologic classification of in utero urinary tract dilatation is presented. Ninety-two hydronephrotic fetal kidneys diagnosed with ultrasound were graded according to the proposed classification. The findings suggest that grade I dilatation (anteroposterior diameter of the renal pelvis less than 10 mm) should be considered normal. Grades II and III constitute an intermediate hydronephrosis, requiring postnatal urologic surgery in nearly half the cases. Grade IV (moderate dilatation of the calyces, with easily identified residual renal cortex) and grade V (severe dilatation of the calyces with atrophic cortex) are clearly pathologic and require neonatal corrective surgery. It is hoped that use of this simple and practical classification will facilitate communication and comparison of results in the literature.