In order to clarify the pathophysiology of dementia due to multiple infarction in the territory of the perforating artery, the reactivity of cerebral vessels to increased carbon dioxide tension was examined in patients with multiple cerebral infarction with or without dementia. The subjects studied were 11 patients with multi-infarct dementia (MID) (age 57-82 years old, mean +/- S.D. 72 +/- 8) and 16 patients with multiple infarction without dementia (MI) (age 51-81 years old, mean +/- S.D. 69 +/- 9). The diagnosis of cerebral infarction was based on the clinical signs and symptoms and findings of magnetic resonance imaging (MRI). Only patients with cerebral infarction located in the perforator territories were included in this study. Dementia was diagnosed by DSM-IIIR criteria. The extent of periventricular high intensity area (PVH) on the T2-weighted image of MRI was classified into 3 subgroups by the criteria of Gerard et al with some modifications. Cerebral blood flow (CBF) was measured by the 133Xe intravenous injection method using a Cerbrograph (Novo), and gray matter flow (F1) and initial slope index (ISI) were calculated. The cerebrovascular reactivity to CO2 was estimated as the increase in F1 or ISI per unit increase in PaCO2 (delta F1/delta PaCO2 or delta ISI/delta PaCO2, respectively) during inhalation of 5% CO2 and as %increase in F1 or ISI per unit increase in PaCO2 (delta F1%/delta PaCO2 or delta ISI%/delta PaCO2, respectively) during inhalation of 5% CO2. 1. CO2 reactivity in both groups. delta F1/delta PaCO2 in the MI and MID groups were 3.2 +/- 1.4 ml/100 g/min/mmHg and 2.0 +/- 1.4, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)