Clinicians completed interviews using the Ohio State University Traumatic Brain Injury Identification Method (TBI-ID) and sections of the Addiction Severity Index (ASI-Lite) with clients who were homeless and receiving community mental health services. Fifty-six percent of clients screened positive for TBI. Clients with a history of TBI were more likely to be using multiple substances, and were more likely to have, and be bothered by, family and emotional problems than clients without TBI. Homeless clients receiving mental health services may have undiagnosed TBI that could affect their ability to benefit from treatment. Such clients may require services that compensate for their cognitive deficits.