A study was made of 32 cases of single ectopic ureters (without duplication) to assess the abnormalities observed in the corresponding renal parenchyma and to correlate renal status with location of the ectopic uretic opening. Specimens were obtained from 26 nephrectomies and renal dysplasia was found in 22 cases. There was a close relationship between renal anomalies and location of the ureteric opening. The more remote the ectopic orifice from its normal position, the more severe were the associated renal lesions. Dysplasia was always present when the ectopic ureter opened outside the urinary tract (vagina, Gartner's duct, vestibule, seminal tract). Am embryological concept is proposed to explain the association of renal dysplasia with abnormal location of the ureter opening. It is based on the abnormal--premature or late--appearance of the ureteral bud on the Wolffian duct resulting in the abnormal positioning of the bud on the Wolffian duct with the ureter growing into nephrogenic tissue lacking the potential for normal renal development and leading to renal dysplasia.