Neurologic disease was the harbinger of the acquired immunodeficiency syndrome (AIDS) in 26 (20%) of 132 patients and ultimately developed in 83 (63%). The most common neurologic disorder heralding AIDS was toxoplasmic encephalitis (17 cases) followed by cryptococcal meningitis (six), retinitis (three), progressive multifocal leukoencephalopathy (two), neuropathy (one), myopathy (one), and subacute "viral" encephalitis (one). In four patients, more than one neurologic disorder coexisted at presentation. Recognition of underlying human T cell lymphotropic virus type III (HTLV-III) infection may allow a more expedient determination of the cause of the neurologic disease. Our study emphasizes the importance of testing for the presence of HTLV-III antibody and assessing the immunologic status of all patients at risk for AIDS who have neurologic illness.