Little is known about the longitudinal course of neuropsychological abnormalities among population-based samples of Human Immunodeficiency Virus infected individuals (HIV1+). While such abnormalities have been studied cross-sectionally in relation to underlying neurological disease, we lack systematic research examining the extent to which they may be independently produced by psychiatric disorders, and about whether--in the presence of established neurological and/or psychiatric disorders--other factors (e.g., HIV1-related medical and psychosocial factors) moderate risk to display neuropsychological deficits. This longitudinal study will evaluate the occurrence, course and predictors of neuropsychological abnormalities in HIV1+ gay/bisexual men seen in primary care medical settings in Allegheny County, PA. A brief neuropsychological screening battery will be administered in the offices of community-based physicians to identify all HIV1+ gay/bisexual male patients who are at high risk to have a cognitive impairment. A random sample of 15% of HIV1+ gay/bisexual male patients at low risk to have such an impairment will be recruited. A second comparison group will include 140 isociodemographically similar HIV1- gay/bisexual men. All of these HIV1+ patients will receive detailed baseline, 6- and 12-months evaluations: Neuropsychological testing, neurological and psychiatric diagnostic evaluations, assessment of predispositional (e.g., medical history) and psychosocial status (e.g., education, substance abuse, perceived stress), historical and current health data (from medical record reviews), and laboratory studies (i.e., T-Cell phenotypes, NK T-Cell activity, p24 antigen). We will have access to similar neuropsychological, neurological, psychiatric, and health-related data from the HIV1- cohort. Data analyses will follow from a conceptual model of the neuropsychiatric predictors of presentation and the course of cognitive abnormality, and of the extent to which the abnormality is itself a critical marker for the subsequent course of HIV1-related physical health.