Computed tomographic (CT) manifestations of cerebral infarction along the distribution of the basal perforating arteries were reviewed in correlation with cerebral angiography. Infarcts in the territories of perforators were demonstrated individually based on knowledge of their three-dimensional distribution as demonstrated by microangiography of cadavers. In Part I of the study, the areas supplied by the medial (MSA) and lateral striate arteries (LSA) were examined. Infarction along the branches of the MSA usually involved the antero-inferior portion of the corpus striatum, immediately posterolateral to the most inferior part of the frontal horn of the lateral ventricle. Infarcts along the branches of the LSA abutted the territory of the MSA posteriorly and superiorly and involved the posterolateral region of the corpus striatum. Clinical and neuroradiological correlations are discussed.