To elucidate whether high blood pressure in acute brain infarction should be treated or not, we retrospectively investigated relationship of the use of antihypertensive agents in the acute stage with the area of hypodensity on CT examined one month after onset and functional outcome at discharge. Actual blood pressure value and diurnal fluctuation in the acute stage were also related to the infarct size and outcome. Subjects were compared of 32 atherothrombotic stroke patients with unilateral severe stenotic lesion (> or = 75% stenosis or occlusion) at the extracranial carotid artery or the horizontal portion of the middle cerebral artery confirmed by cerebral angiography. Seven patients (treated group) received antihypertensive drugs within 14 days of onset and the remaining 25 patients (non-treated group) did not. The data of frequent blood pressure measurements were available in five of the treated group and 24 of the non-treated group. There were no significant differences in the daily highest and lowest mean arterial blood pressure (MABP) between the two groups (124.8 +/- 11.8 mmHg vs. 118.5 +/- 13.5 mmHg and 101.9 +/- 10.4 mmHg vs. 104.4 +/- 12.6 mmHg, respectively). However, apparent circadian fluctuation of the MABP (the highest MABP--the lowest MABP > 25 mmHg) was more frequently seen in the treated group (3/5, 60%) than in the non-treated group (0/24, 0%) (chi 2 test, p < 0.01). Large infarcts (> or = 10 cm2) were more commonly found in the treated group (4/7, 57%) than in the non-treated group (1/25, 4%) (chi 2 test, p < 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)