We studied the relation between blood pressure and type of cerebral infarction (large or lacunar) in 134 patients with acute hemispheral infarcts that were detectable by computed tomography and that could not be attributed to causes other than atherothrombotic arterial disease. Lacunae were present in 26 patients, and systolic blood pressures were higher in this group than in the 108 patients with large infarcts. The overlap was so wide, however, that large infarcts predominated at every level of blood pressure. The presence of a lacune can be inferred only from the combination of clinical signs and, most important, computed tomography.