Acquired renal cystic disease occurs in up to 80% of patients on chronic dialysis and is complicated by renal adenocarcinoma in about 6% of cases. This report suggests that acquired cystic kidneys weighing more than 150 g are six times more likely to contain carcinoma than are kidneys of smaller size. This relationship is true whether or not the large kidney contains a radiologically detectable tumor. Therefore, even in the absence of demonstrable tumor mass, strong consideration should be given to the removal of large acquired cystic kidneys.