Ten per cent of all new cases of AIDS in the United States are in persons older than 50 years. This is particularly problematical in the case of the neuropsychiatric consequences of HIV, because there are neuropsychiatric disorders which become common in older individuals in the absence of HIV. The purpose of this report is to describe the prevalence and incidence of cognitive impairment in HIV-infected individuals enrolled in a community-based study. The study consisted of community-based, sentinel survey physician referrals of HIV-infected patients, with volunteer recruitment of risk-appropriate seronegative controls. One-year longitudinal follow-up study. Detailed neuropsychiatric evaluations were performed at study entry and after one year. A brief, interim visit tracked incident change. Each subject's neuropsychological test performance was classified as normal, demented, or cognitive impairment (not demented). The prevalence of cognitive disorder among HIV-positive individuals over 50 years was significantly greater than in individuals younger than 50 years. Among older participants, dementia was the more common classification (23%), whereas among younger participants, a milder form of cognitive impairment was more prevalent (22%). Alcohol abuse/dependence was a significant risk factor for a disorder, whereas greater education was a protective factor. The one-year incidence of disorder in the sample overall was low (7.3%), and age was not a significant risk factor. However, HIV viral load at study entry was significantly higher among those participants who had developed cognitive impairment one year later. Age is a significant risk modifier for prevalent neuropsychological disorder.