To explore temporal dynamics of cerebral herniation through the calvarial defect after decompressive craniectomy. To investigate patterns of hemispheric asymmetry in ischemic stroke and traumatic brain injury after decompressive craniectomy.To assess clinical implications of hemispheric asymmetry evaluation in order to minimize cranioplasty complications. Using a CT semiautomatic segmentation system, the ipsilateral and contralateral hemispheric areas of patients who underwent decompressive craniectomy for malignant ischemic stroke and traumatic brain injury were measured during the acute (1–7 days), subacute (8–21 days) and chronic (over 21 days) periods. Difference between the two hemispheric areas, called hemispheric asymmetry, has been investigated. Of the 53 patients, 38 (71.7%) had a malignant ischemic stroke, and 15 (28.3%) had a severe head brain injury. In stroke, a significant increase in hemispheric asymmetry was found during acute and subacute phases (+ 6.7 ± 5.1 cm2 and + 7.5 ± 7.2 cm2, respectively; p = 0.002, p = 0.01). An increased hemispheric asymmetry at the time of cranioplasty was associated with a greater risk of complications (p = 0.01). Ischemic stroke and traumatic brain injury exhibit different patterns of brain herniation through the calvarial defect after decompressive craniectomy. There is a greater amount of hemispheric asymmetry in stroke than in trauma, with a peak that is reached during the subacute phases. To minimize cranioplasty complications, it is advisable to wait for the rebalancing of the hemispheric asymmetry.