BACKGROUND Acquired brain injuries (ABI) such as traumatic brain injury (TBI) or stroke can result in motor, language, or cognitive impairments. Although a considerable number of studies have investigated functional recovery, underlying brain reorganization remains poorly understood. Accumulating evidence indicates that plastic processes in the brain are linked to changes in electroencephalographic (EEG) slow wave activity (SWA) during deep sleep (EEG spectral power 1-4.5 Hz). OBJECTIVE We investigated sleep SWA in children and adolescents with ABI. METHODS We used high-density EEG (128 electrodes) to record sleep in 22 young patients with ABI (age range = 4-16 years). We compared patients to 52 previously measured typically developing children and adolescents (age range = 4-16 years). RESULTS The pattern of alterations in SWA differed between particular patient groups. In patients with bilateral stroke, SWA was globally reduced across the entire scalp. Patients with unilateral stroke showed a local reduction in SWA over lesion areas and an increase over perilesional and contralateral brain areas. In patients with severe TBI, we found a reduction in SWA over the midline and an increase over lateral brain areas. We found no consistent pattern in patients with mild to moderate TBI. CONCLUSIONS Sleep SWA seems to be a sensitive measure to assess individual alterations in neural activity after ABI. Deviations from age norms might indirectly indicate plastic processes that have occurred since injury. Improving our understanding of neural activity after ABI could optimize clinical prognosis and guide the development of novel therapeutic interventions.