To evaluate the association of normalized and absolute ADC metrics with progression free survival (PFS) and overall survival (OS) in patients treated for glioblastoma multiforme (GBM). Fifty-two patients with preradiotherapy diffusion weighted imaging treated with post-operative chemoradiation for GBM were evaluated. Region of interest analysis for ADC metrics including mean and minimum ADC value (ADCmean) and (ADCmin) was performed within the T2/FLAIR volume. Normalized (N)ADC values were generated relative to contralateral white matter. PFS and OS were analyzed relative to ADC parameters using a regression model. Kaplan-Meier and Cox proportional hazards analysis with respect to (N)ADCmean, and (N)ADCmin was performed. A (N)ADC threshold <1.3 within the T2/FLAIR volume was analyzed with respect to PFS and OS. Regression analysis indicated that normalized ADC values provide the strongest association with PFS and OS. Kaplan-Meier analysis revealed a non-significant trend toward inferior PFS and OS associated with (N)ADCmean <1.7, and a significant decrement to PFS and OS associated with (N)ADCmin <0.3. (N)ADCmin was a significant prognostic factor when taking into account age, performance status, and extent of resection. ADC thresholding analysis revealed that a retained volume of >0.45 cc per mL FLAIR volume was associated with a trend toward inferior PFS and OS. In the post-operative, pre-radiotherapy setting, the (N)ADCmin is the strongest predictor of outcomes in patients treated for GBM. ADC thresholding analysis indicates that a large volume of normalized ADC value <1.3 may be associated with adverse outcomes.