A 3-year-old boy presented to our hospital with a big abdominal circumference since he was born. He had no history of urinary tract infection or flank pain. The abdominal examination showed a defined cystic abdominal mass with a smooth surface measuring 15 × 10 cm. The abdominal ultrasound revealed a separated acoustic dark area on the left abdomen and bilateral hydronephrosis with upper ureter dilatation on the right abdomen. Similarly, abdominal computed tomography (CT) scan demonstrated a giant ureter on the left side and right hydronephrosis with the whole dilatation of right ureter. Contrast-enhanced CT scan further showed renal dysplasia with a giant ureter. In addition, a dynamic diethylene triamine pentaacetic acid (DPTA) radionuclide renogram showed no function in the left glomeruli and compensatory increase in the right glomeruli. On cystoscopy, the left ureteric orifice could not be found. Based on these examinations, a diagnosis of left CGM causing a malfunction of the left kidney and bilateral hydronephrosis was made. At one-stage of the operation, the giant left ureter and the right ureter dilated about 5 cm from the entrance of the bladder (the submucosal segment of the ureter) were found in the deep right bladder. So we considered that the right ureter was compressed by the giant left ureter, and then a left nephrostomy with a right ureterolysis were performed. After the first operation, the liquid outflowing from the single J tube was about 10 mL per day. After the first operation for 19 days, a dynamic DPTA radionuclide renogram was performed again and revealed a serious decline in the function of left kidney. In addition, an intravenous pyelography showed no images of the left kidney and ureter. These results indicated a poor left kidney function and we considered that the left kidney could not be kept any more. As a result, a second-stage operation was performed thirty days after the first operation. During the operation, we could see a dysplastic left kidney and an almost entirely dilated left ureter with only 1 cm stricture at the entrance of the bladder, then nephroureterectomy was performed through cutting off the left kidney and ureter close to the bladder. The postoperative pathologic examination showed that the left kidney and ureter were similar to multicystic dysplastic kidney. The patient recovered well and remarkably reduced right hydronephrosis was found by the follow-up abdominal ultrasound. The patient was observed to be asymptomatic after 2 years of follow-up.