Records of 37 consecutive patients with acute brain infarction from vertebrobasilar artery disease, admitted to the cerebrovascular hospital service within 36 hours of the onset of symptoms, were studied to define the temporal profile of clinical events during the first week of illness. A stable course with unchanged neurologic deficit was found in 11% (4 patients). Progressive improvement in symptoms occurred in 35% (13 patients). Progressive disability occurred in 43% (16 patients). A remitting-relapsing course was noted in 11% (4 patients). In these latter 2 groups, stabilization of the clinical course occurred in the majority with 48 hours after the initial symptom; however, definite changes continued to develop up to 96 hours in a significant number of patients. Mortality at the conclusion of the first week was 27% for the entire group. This was 2.5 times greater than the 10.6% previously reported for a group of 179 patients with acute brain infarction due to carotid system disease who were concomitantly studied.