We describe a case of an untreated female patient monitored over 8 years for chronic B-lymphocytic leukaemia (B-CLL). Over the 8 years, the patient has gradually developed severe kidney failure, even though the criteria for B-CLL treatment had not been fulfilled. Kidney biopsy revealed renal damage due to lamda free light chains cast nephropathy as well as an infiltration of renal parenchyma with B-CLL cells. It was not before this biopsy that the presence of monoclonal immunoglobulins has been investigated. Immunofixation identified free monoclonal lamda light chains in the serum and urine. Their serum concentration, quantified by densitometry, was 2.6 g/l and urine concentration was 0.5 g/l. A specific evaluation of free light chains in the serum revealed an extremely high concentration of free X light chains, over 4500 mg/l, and normal concentration of K free light chains, 10 mg/l. The aim of this report is to emphasise that monoclonal immunoglobulin may be present in B-CLL as well as other lymphoprolipherative diseases and that it may cause damage to organs, similar to multiple myeloma or monoclonal gammopathy of undetermined significance. The described case confirms poor prognostic value of monoclonal immunoglobulin free light chains in patients with B-CLL and usefulness of an evaluation of their presence in patients with B-CLL, particularly if the patients have increased creatinine level. The described case also highlights the need for evaluation of the presence of free light chains in the serum of all patients with unclear cause of renal failure.