Adenosine deaminase activity (ADA) determination in cerebrospinal fluid (CSF) is considered a specific test for the diagnosis of tuberculous meningitis. In order to study the variability of this marker in patients with different neurological disorders associated with HIV infection, and its utility for the diagnosis of tuberculous meningitis in these patients, the ADA levels in 417 CSF samples from HIV-infected patients with neurological symptoms were reviewed. HIV infection, HIV-associated neurological disorders, and progressive multifocal leukoencephalopathy were not associated with elevated ADA in CSF. Among patients with meningitis, receiver operating characteristic curve analysis gave an optimal ADA cut-off point of 8.5 IU/l for the diagnosis of tuberculous meningitis, with 57% sensitivity, 87% specificity, and an area under the curve of 0.747 (similar to that for CSF glucose concentration). False-positive results were found in patients with neurological CMV disease and cryptococcal, lymphomatous, and probable candidal meningitis. The results of this study indicate that ADA determination in CSF has limited utility for the diagnosis of tuberculous meningitis in HIV-infected patients.