The blood rheology of EDTA-anticoagulated blood samples from blood donors and subjects considered to have myalgic encephalomyelitis was assessed by multiple shear rate viscometry and by multiple-pressure filterability. Although average viscosities of the two groups were different, the differences did not reach statistical significance. In contrast, the data from multiple-pressure filtration of whole blood showed significant differences between females at the lowest (2.5 cm of water) filtration pressure. It appears that the acute phase of the disorder is associated with changes in blood rheology which could impair microcirculatory blood flow. In contrast, the chronic state does not appear to be associated with rheological abnormalities.