A 58-year-old man presenting with increasing shortness of breath over several months is reported here. All investigations were repeatedly normal until he suddenly and rapidly deteriorated, warranting admission, and a diagnosis was made of early multiple myeloma associated with the hyperviscosity syndrome. The plasma viscosity was extremely high compared to the low concentration of paraprotein present, and the possible mechanisms are discussed. Although uncommon, the hyperviscosity syndrome is an important cause of dyspnoea which is readily reversible, and should be considered when investigations of cardiac and pulmonary function are normal.
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