Diffusion tensor imaging (DTI) by magnetic resonance imaging (MRI) is now used not only for delineating white matter fiber tracts, but also for assessing the histological characteristics of pathological tissues. Among these uses, predicting the extent or existence of tumor cell invasion into white matter by DTI is under extensive investigation. The previously reported tumor infiltration index (TII) holds great potential for the discrimination of pure vasogenic edema from tumor-infiltrated edema. However, conflicting data are being reported questioning the clinical value of TII. The present investigation reevaluated the utility of TII in patients with meningioma or glioma. We found that TII was unable to discriminate vasogenic from tumor-infiltrated edema. Conversely, detailed voxel-by-voxel comparison of TII and (11)C-methionie PET in the T2-hyperintense area of gliomas showed that TII and (11)C-methionie PET has a positive correlation, suggesting that, although TII is unable to discriminate the cause of edema, the extent of tumor cell invasion into white matter is depicted in gliomas by TII. These data suggest that TII involves both vasogenic and tumor-infiltrated factors, rather than only a single factor. A more intensive investigation is required to reach a complete understanding of TII.