In a period of 12 years, 887 renal failure patients were treated with hemodialysis in our research center. In 512 patients (57.7%) renal cysts were detected by ultrasonagraphy and/or CT. The cysts were apparently acquired during hemodialysis rather than congenital. More important is the fact that in 19 patients with renal cysts tumor subsequently developed. Surgical specimen confirmed that the tumor was malignant in 8 and benign in 11. The mechanism of genesis of renal cyst and tumor in patients under longterm hemodialysis is discussed. The failed kidneys become ischemic and fibrotic and the glomeruli are functionless or completely replaced by fibrotic tissue. Under the situation, the decreased washing-out effect of urinary flow in the renal tubules and tubular obstruction most likely account for cyst formation. The constant chemical and/or physical irritation by the cyst contents combined with the longterm uremia-induced immuno-suppression may contribute to the neoplastic transformation. From the sequence of events, renal cyst may be considered as a pre-cancerous manisfestation.