The effect of renal transplantation on acquired cystic disease of the kidney in patients who have been on hemodialysis for more than 5 years was examined in 7 cases by computer-assisted tomography (CT scan). Almost all acquired cysts disappeared, and the size of the original kidneys decreased remarkably in 2 cases 8--10 months after transplantation. 3 other patients, in whom CT scans were performed only after transplantation, showed contracted scarred kidneys with few or no cysts. The involution of acquired cysts was incomplete in 1 case, in whom the cysts persisted for 3 years and 2 months, despite normal renal function. The last case, who was off hemodialysis only for 4 months, exhibited enlargement of the original kidneys and an increase in the number of cysts. These results suggest that a significant number of acquired renal cysts in dialyzed patients with end-stage kidney disease regress rapidly after successful renal transplantation, but this is not always so.