The objective of this study was to explore the evolution of apparent diffusion coefficient (ADC) values in magnetic resonance imaging (MRI) in normal-appearing tissue of the corpus callosum during the 1st year after traumatic brain injury (TBI), and relate findings to outcome. Fifty-seven patients (mean age 34 [range 11-63] years) with moderate to severe TBI were examined with diffusion weighted MRI at three time points (median 7 days, 3 and 12 months), and a sex- and age-matched control group of 47 healthy individuals, were examined once. The corpus callosum was subdivided and the mean ADC values computed blinded in 10 regions of interests without any visible lesions in the ADC map. Outcome measures were Glasgow Outcome Scale Extended (GOSE) and neuropsychological domain scores at 12 months. We found a gradual increase of the mean ADC values during the 12 month follow-up, most evident in the posterior truncus (r=0.19, p<0.001). Compared with the healthy control group, we found higher mean ADC values in posterior truncus both at 3 months (p=0.021) and 12 months (p=0.003) post-injury. Patients with fluid-attenuated inversion recovery (FLAIR) lesions in the corpus callosum in the early MRI, and patients with disability (GOSE score ≤6) showed evidence of increased mean ADC values in the genu and posterior truncus at 12 months. Mean ADC values in posterior parts of the corpus callosum at 3 months predicted the sensory-motor function domain score (p=0.010-0.028). During the 1st year after moderate and severe TBI, we demonstrated a slowly evolving disruption of the microstructure in normal appearing corpus callosum in the ADC map, most evident in the posterior truncus. The mean ADC values were associated with both outcome and ability to perform speeded, complex sensory-motor action.