OBJECTIVE This study was an attempt to determine whether CT and MRI are comparable or if one is superior to the other in the early detection of ischemic stroke or hematoma. METHODS Patients with acute stroke were sought within 3 hours of onset for clinical examination and prospective evaluation by concurrently performed CT and MRI. Repeated clinical and imaging studies were undertaken when possible immediately after imaging and at 24 hours, 3 to 5 days, and 3 months. The study neurologists were blinded to the results of imaging, as were the study radiologists to the clinical findings. The study radiologists read the scans in sequence, mapping each imaging on standard templates before viewing a later scan. No retrospective revisions of imaging mapping of earlier images were undertaken. RESULTS Sixty-eight patients were recruited within 4 hours and an additional 12 patients within 24 hours. Seventy-five strokes were due to infarction and five to hemorrhage. The median time to first scan was 132 minutes. Although some of the infarctions in 75 patients were detected within 1 hour, the fraction of positive first scans approached an asymptote at 2 to 3 hours. Overall, with the use of conventional non-contrast-enhanced CT and T1- and T2-weighted MRI, neither was superior in the very early detection of either hematoma or infarction. There was a marginally significant correlation between early positive brain imaging and the severity of the stroke. Some patients had initially positive CT and/or MRI scans, but their neurological examination had returned to normal by 24 hours. Overall, CT was better than baseline MRI at predicting 24-hour outcome. After 24 hours, both CT and MR more conspicuously defined the lesion limits than they did at baseline. CONCLUSIONS With the technology available through 1991, neither CT nor MRI proved superior in the detection of the earliest signs of stroke.