A healthy 8-year-old boy was admitted to our hospital with a complaint of abdominal pain. Vital signs were normal, as were blood-tests findings. Urinalysis revealed a significantly elevated level of urinary occult blood, and left renal pelvic dilatation (grade 3 according to the Society for Fetal Urology classification) was detected on abdominal ultrasound. Abdominal contrast-enhanced computed tomography also showed left renal pelvic dilatation, enlargement of the left kidney along with an extrarenal pelvis, and a bright cord-like substance in the left ureter. On the basis of these findings, we diagnosed transient hydronephrosis due to an aberrant vessel in the ureteropelvic junction. The patient's symptoms improved with conservative treatment. Two months later, renal dynamic scintigraphy revealed delayed excretion in the left kidney. Elective left pyeloplasty was performed 8 months after diagnosis, and there has been no recurrence of abdominal pain since. In cases of acute abdomen, a urine test should be performed, and transient hydronephrosis should be considered in the differential diagnosis.