A study was made of 51 duplex kidneys, ureters and bladders to assess the types of abnormalities seen in duplex kidneys. The abnormalities affected ectopic and orthotopic segments equally. The occurrence of renal abnormality was closely correlated to the abnormal locations of the segment's ureteral orifice. When the orifice was displaced either cranially (orifice positions B, C and D) or caudally (orifices G and H) the orthotopic and ectopic segments, respectively, showed more severe hypoplasia and dysplasia. Faulty embryogenesis is proposed to explain these findings and the importance of endoscopic positioning of the ureteral orifice is emphasized in determining whether heminephrectomy will be necessary.