Chronic lymphocytic leukemia (CLL) is a malignancy characterized by clonal proliferation and accumulation of B lymphocytes. Although leukaemic infiltration of the kidney is well recognized in CLL, acute renal failure (ARF) due to leukaemic infiltration is extremely rare. Here we present a case of ARF as the initial manifestation of CLL. The diagnosis was made by a kidney biopsy. Treatment with cyclophosphamide and prednisolone resulted in a completely improved renal function.