The objective of this study was to investigate changes in self-awareness impairments in outpatients with acquired brain injury (ABI) and the effects these changes have on rehabilitation. Participants were 78 patients with ABI (8.3 years post-injury) who followed an intensive outpatient neuropsychological rehabilitation programme. This longitudinal study comprised pre (T1) and post (T2) measurements and a one-year follow-up (T3). Thirty-eight patients completed the study. The main outcome domains were self-awareness, depressive symptoms, psychological and physical dysfunction, and health-related quality of life (HRQoL). Patients were divided into three awareness groups: underestimation, accurate estimation, and overestimation of competencies. Most patients who underestimated their competencies at the start of treatment accurately estimated their competencies directly after treatment (9 out of 11 patients). These patients also exhibited the largest treatment effects regarding depressive symptoms, psychological and physical dysfunction, and HRQoL. Most patients with impaired self-awareness (i.e., overestimation of competencies) at the start of treatment continued to overestimate their competencies after treatment (10 out of 14 patients). These patients exhibited a significant decrease in depressive symptoms but no other treatment effects. The results indicate that changes in outcome are related to changes in awareness, which underline the importance of taking into account different awareness groups with respect to treatment effects.