The reported incidence of positive scans in the presence of cerebral infarction varies between 25 and 81 per cent (1–8). In a previous report (9) of 129 cases, the present authors found 41 per cent positive, with the greatest number of positive scans in the third week postinfarction. The purposes of the present study are several: (a) to better elucidate the type of cerebrovascular lesion which will produce an abnormal brain scan, (b) to more clearly define the post-infarct periods when the scans are negative and positive, (c) to correlate infarct size as judged clinically with the brain scan results, (d) to demonstrate infarcts by scan in the anterior, middle, and posterior cerebral and the basilar-vertebral artery distributions, and (e) to evaluate the brain scan for its value in diagnosis and prognosis. Materials and Method This study analyses 179 brain scans in 152 patients with the diagnosis of cerebral infarction. Scans were performed three to five hours after intravenous injection of Hg203- or Hg197-labeled chlomerodrin. Commercially available scanners with 3″D × 2″ sodium iodide, thallium-activated crystals were used. The patients were routinely given a prior intramuscular injection of nonradioactive sodium meralluride as a renal blocking dose. Of the 179 scans, 121 (including repeat studies in 27 cases) in 94 patients were obtained consecutively on all patients with the diagnosis of cerebral infarction in the Veterans Administration Hospital, Jackson, Miss., between April 1964 and September 1965. This is an enlargement of the VA group of our previous report. Several additional cases from the Veterans Hospital before and after the consecutive series are included for their value as case reports but are not in the VA series. Scans of 58 patients with a diagnosis of cerebral infarct were obtained on review of 700 brain scans from the University of Mississippi Medical Center. These 58 are not identical with the University Hospital group mentioned in our previous report since an equal number has since been deleted and added to the group. The size and location of the lesion were estimated clinically by one of the authors who did not know the scan results. The number of major neurological findings, i.e., paralysis, sensory loss, visual field loss, and aphasia, were taken into account, as was the severity of involvement. The infarcts were then divided into “small,” “moderate,” and "large" divisions in the same manner as Ojemann et al. (3). The scans were interpreted on two occasions by another of the authors who had no clinical information. Results Of the total of 179 brain scans in this series, 72 (40 per cent) were abnormal. If the clinically small lesions are excluded, 65 of 110 (59 per cent) were considered abnormal. This is in fair agreement with the reported incidence by others (1, 3, 4, 7). The results of the scans are summarized in TABLE I according to lesion size (clinically) at the different time intervals. A peak (95 per cent) of positive scans occurs during the second and third weeks post-infarction in the clinically large lesions. Figure 1 is a graphic representation of some of the data in TABLE I, showing the percentage of positive brain scans for the three clinical size groups related to time. This demonstrates (a) the delay between onset of clinical symptoms and the positive brain scan; (b) essentially the same post-infarction interval for negative scans in all groups, (c) the correlation between clinical size of scan and its appearance on the scan, and (d) the longer period of abnormal scans in the larger lesions.