After reviewing the records of 130 cases of primary hydronephrosis in children, it was found that hydronephrosis occurred almost twice as frequently in boys as in girls. About one third of the hydronephroses were found in children from 1 day to 2 years of age. As a rule, new-borns and infants up to the age of 6 months had the more advanced and worst type of hydronephrosis. In this group the presence of contralateral renal anomalies was alsq greater. About 40% of the children with hydronephrosis had another important anomaly present in the genitourinary tract or elsewhere in the body. About 20% of these children had bilateral hydronephrosis. About 50% of the hydronephrotic kidneys, when first discovered, required nephrectomy or nephrostomy. In two thirds of the cases a definite obstructive pathological cause of hydronephrosis was found. What can be expected, from the clinical and pathological standpoints, in a child who