OBJECTIVE: To determine characteristics of patients with complicated mild traumatic brain injury (CMTBI) on the inpatient rehabilitation unit and to accentuate limits of current classification systems for patients with mild TBI. DESIGN: Multicenter analysis of individuals with a Glasgow Coma Scale (GCS) score of 13 to 15 (lowest score in 24 h) and positive computed tomography findings admitted to inpatient rehabilitation for TBI. SETTING: 16 TBI Model System centers funded by the National Institute on Disability and Rehabilitation Research. PARTICIPANTS: 373 adults with CMTBI. MAIN OUTCOME MEASURES: Functional Independence Measure (FIM) and Disability Rating Scale (DRS) scores, length of stay, and medical complications. RESULTS: 16% of the sample experienced serious cranial complications (eg, cerebrospinal fluid leak, intracranial hypertension), with seizures being the most common during the course of inpatient rehabilitation. Most patients demonstrated substantial improvements on functional outcome measures during rehabilitation (mean admission FIM 68.2, discharge FIM 100.6; admission DRS 9.1, discharge DRS 4.8). Age, acute length of stay, and FIM score on rehabilitation admission were significant predictors of rehabilitation length of stay. Factors not predictive of rehabilitation duration were GCS score, ethnicity, gender, incidence of fractures, and incidence of cranial/noncranial complications. CONCLUSION: Despite favorable GCS scores, some patients with CMTBI suffer a variety of serious cranial and noncranial complications. Such patients appear to benefit from additional brain injury services offered in inpatient rehabilitation. Further study of the unique treatment needs of these patients is warranted. © 2006 Lippincott Williams & Wilkins, Inc.