We aimed to determine the sensitivity of CT perfusion (CTP) for the diagnosis of cerebral infarction in the acute stage. We retrospectively reviewed patients with ischemic stroke who underwent brain CTP on arrival and MRI-diffusion weighted image (DWI) after hospitalization between October 2008 and October 2011. Final diagnosis was made from MRI-DWI findings and 87 patients were identified. Fifty-five out of 87 patients (63%) could be diagnosed with cerebral infarction by initial CTP. The sensitivity depends on the area size (s): 29% for S < 3 cm(2), 83% for S ≥ 3 cm(2) - < 6 cm(2), 88% for S ≥ 6 cm(2) - < 9 cm(2), 80% for S ≥ 9 cm(2) - < 12 cm(2), and 96% for S ≥ 12 cm(2) (p < 0.001). Sensitivity depends on the type of infarction: 0% for lacunar, 74% for atherothrombotic, and 92% for cardioembolism (p < 0.001). Sensitivity is not correlated with hours after onset. CT perfusion is an effective imaging modality for the diagnosis and treatment decisions for acute stroke, particularly more serious strokes.