Histories and computed tomograms of 606 patients with transient cerebral ischaemia were studied. All symptoms and signs had completely resolved within 24 hours, and any episodes suggestive of posterior fossa ischaemia were excluded. Computed tomography, done after the clinical features had resolved, showed 79 relevant infarcts: 46 were small, deep, lacunar infarcts (58%, 95% confidence interval [CI] 47-69%), and 33 were larger cortical infarcts. The histories and the type of infarct in these 79 patients were compared to see whether lacunar infarcts were preceded by a history of unilateral motor or sensory symptoms without features usually attributed to the cerebral cortex. The positive predictive value of such lacunar symptoms was 0.74, with a negative predictive value of 0.61. 11 patients had a cortical infarct despite a history of lacunar TIAs, but only one occurred in the left hemisphere and speech was not affected. Of 527 patients with transient ischaemic attacks without a relevant infarct visible on computed tomography, 335 (64%) had a history suggestive of lacunar ischaemia, whereas in several other studies 20-25% of patients with ischaemic stroke have evidence of lacunar infarcts. Lacunar TIAs may therefore have a better prognosis than cortical TIAs or may often precede cortical infarcts; alternatively, many cortical infarcts may occur without warning.