PURPOSE: To assess the usefulness of MRI in the detection of a single ectopic ureteral opening and thelocation and dysplastic change of ipsilateral kidney. MATERIALS AND METHODS: Nine patients (mean age; 4.8 years,M:F=3:6) in whom a single ectopic ureter was suspected clinically and sonographically underwent conventionalradiologic studies ( IVP, VCUG, 99mTc-DM-SA scan, as well as US) and MRI. We evaluated images of the point of theectopic ureteral opening and the location and dysplastic or hydronephrotic change of the ipsilateral kidney, andcompared those findings with the endoscopic, surgical, and pathological findings. RESULTS: Eight patients had aunilateral single ectopic ureter and one had bilateral lesions. Seven normally positioned kidneys in six patientsshowed dysplastic (n=3) or hydronephrotic (n=4) change. In two patients an ectopic dysplastic kidney was locatedin the pelvis and one had ipsilateral renal agenesis. Conventional radiologic studies failed to reveal twoectopic dysplastic kidneys, one renal agenesis, and eight ectopic ureteral openings. In all patients, MRI clearlydemonstrated the location of the kidney and ectopic ureteral opening, and dysplastic or hydronephrotic change ofthe kidney, and in one patient, uterine duplication. Except in two patients whose ectopic ureteral opening was notfound on endoscopy, MRI findings were concordant with endoscopic and surgical findings. CONCLUSION: MRI wasuseful for the detection of a single ectopic ureteral opening and for demonstrating the location and dysplasticchange of ipsilateral kidney.