We report a 2-month-old boy who presented with severe hyponatremia and hyperkalemia secondary to ureteropelvic junction(VPJ) obstruction. By prenatal ultrasonography at 19 weeks of gestation, severe hydronephrosis was found which was confirmed postnatally Pyeloplasty was done on the 45th day of life, and fifteen days after pyeloplasty, non-bilious vomiting, decreased activity and dehydration developed. Severe hyponatremia and hyperkalemia were observed, as a result of elevated serum aldosterone and plasma renin activity. The anterior posterior pelvic diameter(APPD) and Society for Fetal Urology(SFU) grade measured showed no interval change before and after pyeloplasty. Pseudohypoaldosteronism was diagnosed, and 2M NaCl was administrated orally for 7 days. The electrolyte imbalance was corrected, and 8 weeks later, the elevated levels of aldosterone and plasma renin activity were normalized. The left hydronephrosis was improved at 5 months of age. We hereby report a transient pseudohypoaldosteronism secondary to UPJ obstruction with a review of the literature.