Very large bilobed cystic mass arising in the pelvis with gross hydronephrosis and hydroureter right > left. Mass has 2 lobes: on the left is thick-walled and trabeculated (containing a calculus) consistent with the bladder and on the right is thin-walled and communicates with the bladder via a defect in the wall = pseudodiverticulum. Left leg swelling is due to extrinsic compression of the left iliac veins.