Lacunar infarctions are small lesions caused by occlusion of the deep and penetrating vessels. Occlusion of such vessels are resulted from thrombosis based on atherosclerotic change. However, we occasionally encounter a patient with lacunar infarction seemed to be caused by embolic mechanism, as who has had previous experience of TIA or presented with clinical symptoms suddenly. Diffusion-weighted imaging (DWI) has high accuracy for depicting small ischemic lesions and discrimination of recent infarctions from old ones. We studied frequency, risk factors and stroke mechanism in the territory of penetrating arteries detected on DWI, and sensitivity of ischemic lesions by DWI as compared with conventional MRI images. Fifty-three consecutive patients with lacunar infarction in the territory of penetrating arteries who admitted to our hospital and were studied by DWI within 24 hours. Nine (17.0%) of 53 patients had multiple high signal lesions on DWI. These lesions were found in single arterial territory in 4 patients and in more than 2 territories in 5 patients. It was presumed that stroke mechanisms were artery to artery or cardiogenic embolism. DWI revealed all 38 hyperintensity lesions, while conventional MRI revealed 35 lesions, although the latter was resulted from retrospectively referring to the DWI finding. Therefore, conventional MRI alone might have been difficult to detect multiple lesions. DWI was enable to distinguish embolic infarction from small vessel lacunar infarction, leading to feasible patient management. Therefore, DWI should be performed in all patients with lacunar infarction, even if neurological finding is correspondent with the conventional MRI finding.