We determined in 725 sequentially admitted first-ever stroke patients, using color-coded duplex flow imaging of the V2 segment, whether vertebral artery hypoplasia (VAH) (diameter < or = 2.5 mm) was more frequent (13%) in posterior circulation territory infarction (247) than in strokes in other territories (4.6%). This difference is significant (p < 0.001), whereas all other risk factors (hypertension, hyperlipidemia, diabetes, smoking) were equally (p > 0.05) distributed. Patients with VAH may be predisposed to stroke in the posterior circulation.