We investigated 100 consecutive cerebral ischemia patients for hemorheological alterations. We measured whole and adjusted blood viscosity at 75 and 1,500 sec-1, plasma viscosity, red blood cell aggregation by the zeta sedimentation ratio, and red blood cell deformability using the centrifugal deformability technique. Patients were studied within 72 hours of the acute ischemic event, and 66 were available for follow-up evaluation approximately 2 months later. Two age- and sex-matched control groups were evaluated: 20 nonvascular neurological inpatients (patient controls) and 45 normal volunteers (normal controls). Compared with normal controls, we found significant acute increases in whole blood viscosity (1,500 sec-1), plasma viscosity, fibrinogen concentration, and zeta sedimentation ratio; the latter two variables were also increased at follow-up. Fibrinogen concentration was significantly associated with zeta sedimentation ratio and plasma viscosity and was increased for patient controls. There was a trend toward normalization of acute abnormalities over the 2-month follow-up period, and patients with more severe strokes tended to have more extensive hemorheological abnormalities. Among patients with severe stroke, fibrinogen concentration was significantly associated with the platelet activation peptide beta-thromboglobulin acutely (r = 0.63, p less than 0.005). We conclude that hemorheological abnormalities in cerebral ischemia are largely nonspecific findings, with the likely exception of patients with severe stroke.