Computed tomography (CT) plays a pivotal role in the diagnosis of acute stroke and in treatment decision making. CT perfusion imaging performed with intravenous iodinated contrast material allows calculation of the time to peak enhancement, mean transit time, and cerebral blood volume, important parameters for differentiating between an ischemic penumbra, which might benefit from intravascular therapy with thrombolytic agents, and infarcted tissue, which would not benefit from such therapy. Differentiation between the two entities is important because thrombolytic therapy is associated with an increased risk for intracranial hemorrhage. A finding of delay in peak enhancement or increased mean transit time in a region with normal or only slightly abnormal cerebral blood volume is suggestive of an ischemic penumbra; however, accurate interpretation of the CT perfusion parameters may be difficult in the presence of a cerebrovascular anatomic variant or physiologic condition that produces benign oligemia leading to a false appearance of penumbra. For this reason, CT perfusion parameters must be correlated with the clinical history and findings at unenhanced head CT, angiography or CT angiography, and diffusion-weighted magnetic resonance imaging. The authors identify five possible causes of false penumbras, each of which produces a different pattern at imaging: upstream flow restriction, evolution of ischemic change, vascular dysregulation, positioning of the patient's head at an angle during image acquisition, and variant anatomy in the circle of Willis. Familiarity with the imaging patterns and causes of false penumbras may increase the radiologist's confidence in diagnosis and help avoid costly errors in treatment.