Patients enrolling in one of several AACTG-sponsored potent antiretroviral therapy trials (a "parent" trial) may enter this study. [AS PER AMENDMENT 06/06/00: Patients already enrolled in an AACTG-sponsored study who are changing treatment due to virologic failure may also enter this study.] [AS PER AMENDMENT 11/15/01: Patients starting a new potent antiretroviral regimen as part of their clinical care, enrolling in a potent antiretroviral treatment trial, or changing potent antiretroviral therapy in clinical care or in an ongoing antiretroviral treatment trial because of virologic failure may enter this study.] Patients receive no treatment but undergo various procedures aimed at characterizing the effects of antiretroviral therapies on CSF viral load and cognitive function. Procedures include: 1) venipuncture to measure plasma HIV-1 RNA and DNA levels, CD4+ T cell count, and cytokine and immune activation markers associated with HIV-1 neurological disorders; 2) neuropsychological examinations to measure cognitive function; and 3) lumbar punctures to obtain CSF samples, which are used to determine the pharmacokinetics of antiretroviral agents in CSF and to determine levels of blood cells, cytokine and immune activation markers, and HIV-1 RNA and DNA. An entry visit must occur before initiating potent antiretroviral therapy in the parent trial [AS PER AMENDMENT 06/06/00: or before changing the antiretroviral regimen due to virologic failure in an ongoing trial]. [AS PER AMENDMENT 11/15/01: Patients are registered before initiating a new potent antiretroviral regimen.] Subsequent visits occur within 21 days prior to each lumbar puncture and at Weeks 24 and 52. If evaluations, procedures, or assays for a given patient's parent trial [AS PER AMENDMENT 11/15/01: for any coenrollment AACTG study] occur at the times specified in this study, they are not duplicated for this study. Other visits may occur when a patient changes antiretroviral treatment or discontinues a parent trial [AS PER AMENDMENT 11/15/01: discontinues a potent antiretroviral therapy].