We describe a 51-year-old woman with a cerebral infarction that showed transient accumulation of thallium-201. On admission, this lesion was well-enhanced by gadolinium injection and gradually expanded, mimicking a malignant brain tumor. A cerebral angiogram, however, did not indicate the presence of a malignant brain tumor. Ethyl cysteinate dimer single photon computerized tomography showed perfusion defects throughout hospitalization. The final diagnosis of cerebral infarction was established by pathological examination. Six months after onset, the enhancement by gadolinium and the expansion of the lesion disappeared. A cerebral infarction showing transient uptake of thallium-201, and lesion expansion is indicative of a lesion in the liquefaction stage that might mimic a malignant tumor. Although thallium-201 scintigrams are useful for the differential diagnosis of radiation necrosis and recurrent brain tumor, the findings in this patient should alert clinicians to the differential diagnosis of intracerebral expansive lesions.