Hematocrit values, plasma fibrinogen levels, and erythrocyte deformability are well-recognized factors affecting blood viscosity. We studied the incidence of abnormalities in these factors among 17 patients suffering from thromboangiitis obliterans (Buerger's disease) compared with 14 normal age- and sex-matched control subjects. Hematocrit values and fibrinogen levels were higher, whereas the erythrocyte filterability index was lower among the patients with Buerger's disease (p < 0.001). Incubation of washed erythrocytes from these patients with control plasma of blood type AB produced an improvement of the filterability index (p < 0.005). Our findings suggest that patients suffering from Buerger's disease show, in addition to the well-recognized arterial lesions, an abnormality in the factors affecting blood viscosity. This may contribute to a worsening of the clinical manifestations. Evidence is presented suggesting that a plasmatic factor may be responsible for the impaired erythrocyte filterability.