The posterior pararenal space(PPS) is a potential space between the posterior renal fascia and the transversalis fascia. We reviewed 12 cases of posterior pararenal effusion. The causes of the effusion were retroperitoneal hemorrhage due to trauma(7 cases0 or rupture of renal tumor(1 cases). pancreatitis(2 cases), urinoma(1 case) and amebic colitis of the cecum(1 cases). The CT findings of the effusion were semilunar fluid density in the dependant portion of the PPS(9/12), partial obliteration of the PPS extending to the flank stripe(2/12), and total obliteration of the PPS extending to the anterior abdominal wall(1/12). The effusion in the ipsilateral perirenal space and thickening of the ipsilateral posterior renal fascia were combined in all cases. The effusion in the ipsilateral anterior pararenal space was associated in 11 cases of 12. The effusion in th PPS is not uncommon findings of retroperitoneal hemorrhage or effusion, but usually related with perirenal or anterior arerenal effusion with no clinical significance.