Patients with Waldenstrom macroglobulinemia were studied for the presence or absence of the hyperviscosity syndrome, the relative serum viscosity value, and the calculated whole blood viscosity to identify a level at which symptoms occurred. The majority of symptomatic patients had whole blood viscosity values above 8.0 centipoises. There was a direct correlation between whole blood viscosity and relative serum viscosity, r = 0.75. One patient with central nervous system abnormalities was identified as having a high whole blood viscosity but a low serum viscosity. It was concluded that the vast majority of patients with the hyperviscosity syndrome will be identified by measuring the relative serum viscosity. In patients with central nervous system findings and a low serum viscosity, the whole blood viscosity should be determined either by direct measurement or by calculation.