The hyperviscosity syndrome refers to a group of symptoms and signs related to increased blood viscosity often produced by monoclonal immunoglobulins. It is most frequently associated with Waldenström's macroglobulinemia and, on occasion, with other immunoglobulins that are capable of forming highly polymerized molecules. This article is a report on the first case of pure light chain myeloma associated with the hyperviscosity syndrome. The hyperviscous plasma in this case is secondary to the unusual degree of aggregation of kappa light chain as demonstrated by high-resolution electrophoresis, serum immunofixation, and Sephadex g-200 (Pharmacia, Piscataway, NJ) column chromatography.