The primary goals of this systematic review were to assess studies of homeless adults and cognitive functioning, and to explore the clinical implications and potential impacts on social functioning of these cognitive deficits. The MEDLINE, CINAHL, EMBASE, PsycINFO, Applied Social Sciences Index and Abstracts, ERIC, Social Sciences Abstracts, Social Sciences Citation Index, Social Services Abstracts, and Sociological Abstracts databases were searched from 1970 (or their inception) to October 2007. Abstracts from 582 studies were screened and 22 studies were found to meet inclusion criteria (published in the English language, reported results of neuropsychological tests, or screening tests for cognitive dysfunction in homeless people aged 18 years or older).

Two investigators independently reviewed each study and rated its quality based on well-defined criteria. Results: Ten studies were of good or fair quality. Studies that administered the Mini Mental State Examination indicate about 4% to 7% of homeless people exhibit global cognitive deficits. Focal deficits in verbal and visual memory, attention, speed of cognitive processing, and executive function were also apparent. No studies of the functional implications of cognitive deficits were found.

Cognitive deficits in homeless people likely have a significant mediating impact on the effectiveness of skills training and rehabilitation programs. Clinicians should be cognizant that cognitive deficits probably impair homeless patients' ability to maintain housing stability and follow treatment recommendations. Implications for housing options and health service delivery should also be considered. Research is lacking on interventions to improve cognitive functioning in the homeless.