Long-term dialysis patients frequently develop acquired cystic disease of the kidneys (ACDK). One hundred and sixty-seven dialysis patients were classified into two groups according to the presence or absence of ACDK, and the two groups were compared with regard to the dialysis period, laboratory findings, and so forth. Among the 167 dialysis patients, 37.7% showed ACDK and had an average dialysis duration of 10.3 years, which was significantly longer than the 5.0 years for patients without ACDK. The proportion of patients with ACDK rose with increasing duration of dialysis; 64.6% of the patients on dialysis for more than 10 years had ACDK. The values of hemoglobin and hematocrit of patients with ACDK were significantly higher than those without ACDK. Of 12 dialysis patients who were operated upon because of suspected renal tumor, pathological examination confirmed renal cell carcinoma in 8 and adenoma in one. One of these patients had been treated with chronic ambulatory peritoneal dialysis, and another had bilateral renal tumor. Among the 8 patients with renal cell carcinoma, ACDK was detected in 5 by means of CT and ultrasound tomography, and cystic changes were observed in two on light microscopic examination of the resected kidneys. Pathological examination of the non-tumorous renal tissues revealed hyperplastic or adenomatous changes in the cyst epithelium in 5 patients undergoing dialysis for more than 7.5 years. These changes were thought to be pre-neoplastic. The high incidence of ACDK and the development of renal tumors in long-term dialysis patients indicates the necessity of intensive monitoring.