The most common congenital abnormality of the urinary tract is a duplex kidney. Pelvic-ureteric junction obstruction (PUJO) is a rare association that can affect the lower moiety of incomplete duplex kidneys. We report two adult cases of PUJO of the lower moiety in a duplex kidney that both presented with pyonephrosis. This late presentation of lower moiety PUJO with pyonephrosis has not been described previously. We describe the imaging appearances of this rare association and highlight this important diagnostic consideration in lower moiety hydronephrosis of the adult patient.