In December 2005, the 55-year-old patient was hospitalized because of acute kidney failure and suspected hemorrhagic fever. The physical examination showed splenomegaly (spleen ultrasound-18 cm in large diameter, and 11 cm by palpation) with thrombocytopenia and anemia. He underwent kidney biopsy which described infiltration of small B cell lymphocytes with positive lambda chains. His bone marrow showed infiltration of atypical lymphocytes, and flow cytometry was typical of B-cell CLL. Patient started therapy with corticosteroids (methylprednisolone 80 mg iv) and continued treatment with prednisone (Decortin 20 mg tablets) and chlorambucil (Leukeran 16 mg tablets) through three days. An addition to therapy lead to an increase in platelet count, creatinine level decline and recovery of renal function was observed. He was treated with 6 cycles of therapy with prednisone and chlorambucil and achieved a satisfactory therapeutic effect with adequate hematologic parameters and less severe splenomegaly. Maintenance therapy was continued with prednison at daily dose of 10 mg. Our patient is one of the amongst previously reported as an example of a rare complication of CLL'with leukemic infiltrate causing acute renal insufficiency. Renal biopsy is necessary to confirm the diagnosis. This complication appears to respond well to a variety of treatments. Our patient achieved complete resolution of renal failure and partial hematological response with combination of chlorambucil and prednisone.