Background: Diffuse low-grade gliomas are rare brain tumors transforming to higher grade even with surgery, chemotherapy, and radiotherapy. Their preferential infiltration of white matter tracts, beyond tumor boundaries on fluid-attenuated inversion recovery (FLAIR), make difficult to plan focal treatment such as surgery or radiotherapy and monitor response to chemotherapy. Diffusion tensor imaging (DTI) might reflect this infiltration of white matter tracts. The aim of our study is to assess how DTI signal in the peritumoral zone might be modified before FLAIR tumor progression appears at 1-year follow-up. Methods: The study retrospectively enrolled 5 patients who met inclusion criteria: DTI with 25 directions, T1 and FLAIR at initial imaging; FLAIR at one-year follow-up. Patients with surgery, radiotherapy, and chemotherapy completed less than 2 years before initial imaging were excluded. FLAIR tumor progression, named progression mask, was assessed by subtracting tumor masks between initial imaging and one-year follow-up. Initial DTI signal was analyzed within this progression mask and compared with the healthy contralateral side. Results: Tumor progression was confirmed for the 5 patients at 1 year. All patients showed pre-existing DTI signal abnormalities within the progression mask. Mean fractional anisotropy (P = 0.03) was lower in the progression mask, whereas mean diffusivity, axial diffusivity, and radial diffusivity mean (P = 0.03) was higher in the progression mask, compared with the healthy side. Conclusions: This study shows pre-existing DTI signal abnormalities in regions with tumor progression at 1 year. Such abnormalities could correspond to a tumor infiltration not yet visible on FLAIR. This might be helpful to predict tumor progression and allow to adapt the therapeutic strategy.