Twenty-two patients with ischemic stroke, as a single event, in the territory of basilar artery (BA) are reported. On the basis of the findings from computerized tomography (CT) and clinico-radiologic features, the authors propose that this heterogeneous entity-the basilar artery (BA) syndrome-can be divided into five subtypes. Type 1 (complete type), characterized by infarctions in the whole territory of BA, is incompatible with life; type 2, with extensive brain stem infarct, may result in a locked-in state; and type 3, with infarctions in part of the BA territory (incomplete form or ''partial syndrome'') may have a more variable clinical outcome. However, type 4, with a top of the BA syndrome, and type 5, with negative CT BA syndrome (angiographically verified), are often more benign. Although initial CT scanning may infrequently be unrevealing, serial and follow-up CT scannings have proven their usefulness in the majority of cases as a noninvasive tool, in contrast to cerebral angiography, for predicting the short-term prognosis of BA syndrome.