Object Identifying ischemic lesions after subarachnoid hemorrhage (SAH) is important because the appearance of these lesions on follow-up imaging correlates with a poor outcome. The effect of ischemic lesions seen on computed tomography (CT) scans during the first days of treatment remains unknown, however. Methods In 156 patients with SAH, clinical course and outcome, as well as the appearance of ischemic lesions on serial CT scans, were prospectively monitored for 3 months. At 3 months after SAH, magnetic resonance imaging was performed to detect permanent lesions that had not been visible on CT. Results Of the 53 patients with no lesions on any of the follow-up CT scans, four (8%) had a poor outcome. Of the 52 patients with a new hypodense lesion on the first postoperative day CT, 23 (44%) had a poor outcome. Among the remaining 51 patients with a lesion appearing later than the first postoperative morning, 10 (20%) had a poor outcome (p < 0.001). After adjusting for patient age; clinical condition on admission; amounts of subarachnoid, intracerebral, and intraventricular blood; and plasma glucose and D-dimer levels, a hypodense lesion on CT on the first postoperative morning was an independent predictor of poor outcome after SAH (odds ratio 7.27, 95% confidence interval 1.54–34.37, p < 0.05). Conclusions A new hypodense lesion on early postoperative CT seems to be an independent risk factor for poor outcome after SAH, and this early lesion development may be more detrimental to clinical outcome than a later lesion occurrence.