Relationships among mild traumatic brain injury (MTBI), posttraumatic stress (PTS), and function were examined in 99 motor vehicle accident (MVA) admissions: 64 in an MTBI group and 35 in a no-MTBI comparison group. Assessments occurred within the first month and at 6 to 9 months. At follow-up, the sample was moderately disabled on the Sickness Impact Profile (SIP), 71% satisfied on the Reintegration of Normal Living Index (RNL), and 42% had returned to work. Only the SIP Psychosocial score was significantly different for MTBI groups; 24% of the sample showed definite symptoms of PTS. This group was significantly more disabled on the SIP, less satisfied on the RNL, and less likely to return to work. The proportion of variance in outcome explained in each model ranged from 32% (Physical SIP) to 44% (RNL). Results suggest the need for clinicians to be more aware of the strong influence of PTS on functional outcomes.