We have seen 21 patients with unilateral congenital giant megaureter. Age ranged from 2 months to 8 years. This condition is characterized by giant focal segmental ureteral dilatation producing an elongated and distorted ureter; distally there may be congenital ureteral stenosis or atresia, or a ureter of approximately normal caliber. They usually have a dysplastic and disorganized muscular coat, lined with a columnar epithelial mucosa rather than the usual transitional epithelium. The associated kidney or moiety was hypoplastic, dysplastic, or the site of comparatively mild hydronephrosis associated with atrophic renal parenchyma. Function and volume of the bladder was normal. Twelve of 21 patients had a duplex collecting system on the affected side. One patient had four pelves and proximal ureters opening into a single lower ureter, which was the site of a giant cyst. One of eight simple giant megaureters was suspected of being associated with a solitary kidney. Depending on whether the giant segmental ureteral dilatation was multiple or single, these cases have presented either with marked protruberance of the mid and lower abdomen, which was distended by transverse elongated cysts, or with a round cyst mass in the lower abdomen. Nineteen patients treated by excision of the giant megaureter and the associated kidney or moiety were free of symptoms postoperatively. One 4-month-old infant with multiple congenital abnormalities and urinary tract infections died soon after operation. Another 4-month-old infant with suspected solitary kidney was treated by ureterostomy and was discharged in a critical condition without follow-up.