The severity of traumatic brain injury (TBI) is determined by many variables, the complexity of which has made prediction of functional outcome an elusive target. To evaluate whether the three components of the Glasgow Coma Scale (GCS) and their alterations over time can serve as predictors of functional outcome after a severe TBI at 12 months after the TBI insult, we carried out a prospective study of patients with severe TBI. Seventy patients were initially enrolled. Data were retrieved from the emergency department records and the patients' intensive care unit, neurosurgical, and rehabilitation unit records. All patients underwent follow-up at 3, 6, and 12 months after injury. GCS components were evaluated on the day of injury and 2 weeks after injury. Functional outcome was estimated using the Glasgow Outcome Scale and the Functional Independence Measure motor scale. It was evaluated during rehabilitation and at 12 months after injury. Fifty-one patients were alive and followed up until 12 months. Logistic regression and receiver-operator characteristic curve analyses were carried out. In terms of functional outcome at 12 months, only GCS on day 15 was found to be a prognostic factor, with all its subscales being related to outcome 12 months later, whereas a higher GCS score on day 15 was also related to survival. A higher motor and verbal response on day 15 was strongly associated with a patient's functional independence, whereby the motor response was a better predictor. The GCS motor score 2 weeks after injury was statistically significantly associated with the 12-month functional outcome in TBI survivors. Motor response was the most useful predictor among the GCS components with respect to the long-term functional outcome in patients with severe TBI.