[50-year-old man with fatigue and monoclonal gammopaty].

This article presents an extremely rare case of monoclonal gammopathy of undetermined importance (MGUS) (known for 16 years) that progressed to multiple myeloma during cytotoxic treatment of malignant lymphoma. Morphological and immunochemical studies were used to characterize the two blood diseases. Identification of type 2 (malignant lymphoma) and type 1(multiple myeloma) disease indicate that the condition was biclonal. The malignant lymphoma responded completely to treatment with chlorambucil and the multiple myeloma responded partially to treatment with melphalan and prednisolone some months later. Diagnosis and follow up of MGUS is commented on.