PURPOSE: To evaluate by US and CT the incidence and complications of acquired cystic kidney disease (ACKD) in dialysis and renal transplant patients and to compare the effectiveness of US and CT in the diagnosis of this entity. MATERIALS AND METHODS: This study was prospectively performed in 70 dialysis patients and 13 renal transplant patients, and excluded any with multiple renal cysts or polycystic kidney disease, on as seen on initial films. US were obtained in all patients, and CT scans were randomly obtained in 27 who had been on dialysis for 3 years or more; all these US and CT scans were analyzed, with particular emphasis on whether or not cysts were present. In order to correlate the numbers of cysts with duration of dialysis, all patients were assigned to one of three groups, according to the number of cysts found (group 1, 0; group 2, 1-4; group 3, >4).Only group 3 was diagnosed as suffering from ACKD. In order to compare the cyst-detection capability of US with that of CT, 27 dialysis patients who had undergone US and CT were divided into four groups according to the numberof cysts found (grade 1, 0; grade 2, 1-4; grade 3, 5-10; grade 4, >10). RESULTS: Seventy dialysis patients were divided according to the results of US, as follows : group 1, 20%; group 2, 47.1%; group 3, 32.9%. The mean duration of dialysis in group 1 (31.9 months) was statistically different from that in group 2 (50.6 months) and in group 3 (95.8 months) (p<0.000). Thirteen renal transplant patients were divided as follows : group1, 61.5%; group 2, 38.5%; group 3, 0%. In dialysis patients with ACKD, complications noted were renal cell carcinoma(n=1), hemorrhagic cysts(n=2), and hematomas(n=2) Among the 27 dialysis patients who underwent CT, this and US showed an equal grade of cystic change in 53.7%, while CT showed a higher grade in 46.3%. The detection rate of ACKD in these 27 patients was 46% on US and 63% on CT. CONCLUSION: A prolongation of dialysis corresponded to an increased incidence of ACKD; renal neoplasm and hemorrhage may occur in dialysis patients, but ACKD and its complications did not develop in renal transplant patients. In long-term dyalysis patients, regular follow-up studies of kidneys using US or CT are therefore needed. CT was superior to US in diagnosing ACKD.