The differences between peritumoral brain tissue infiltrated by tumor and vasogenic edema were prospectively evaluated by comparing the apparent diffusion coefficient (ADC) of peritumoral areas of infiltrative tumors (anaplastic astrocytomas and glioblastomas) to that of peritumoral areas of noninfiltrative tumors (metastatic carcinomas) on 54 patients. Peritumoral ADCs indicated the possibility of differentiation between tumor infiltration and vasogenic edema, as well as between primary gliomas and metastases.