The pattern of leukocyte aggregation and its possible relationship with the clinical outcome were evaluated in 35 untreated patients with acute cerebrovascular disease during each of the first three days of hospitalization. Although statistically significant differences were found when comparing patients and age and sex matched controls during any of the 3 days, a large overlapping of values was noted in each comparison. The overlapping was strongly reduced when the average 3-day value was compared. Similarly, when comparing subgroups of patients classified on the basis of their clinical status, the overlap of values observed during any of the 3 days dropped from 70% to 30% when considering the average 3-day levels of leukocyte aggregation. Furthermore, the level of correlation between leukocyte aggregation and size of cerebral infarction was better when considering the average 3-day value (p = .005) than the best single-day correlation (p = .01). These data suggest that the association between acute cerebrovascular ischemia and leukocyte rheology is better discovered when repeated evaluation of the latter are performed.