Highly active antiretroviral therapy (HAART) effectively reduces human immunodeficiency virus (HIV) RNA in cerebrospinal fluid (CSF), as well as in plasma. The effect on intrathecal immunoactivation is less well studied. We had earlier found that a substantial number of patients still have evidence of intrathecal immunoactivation after up to 2 years of treatment. We identified 15 patients treated with HAART for > or =4 years who had plasma HIV-RNA levels of <50 copies/mL for > or =3.5 years. CSF samples were available from 10 patients before treatment. We measured white-blood-cell count, HIV-RNA level, neopterin level, and IgG index. During treatment, all patients had HIV-RNA levels of <50 copies/mL in plasma and CSF. In CSF, both neopterin level and IgG index decreased significantly. After 4 years, 9 (60%) of the 15 patients still had neopterin levels in CSF that were above the upper normal reference value (5.8 nmol/L). During HAART, 9 (60%) of the 15 patients had an abnormal IgG index (>0.63). HAART significantly decreases intrathecal immunoactivation, but, despite effective treatment for >4 years, with HIV-RNA levels <50 copies/mL for > or =3.5 years, a substantial proportion of patients continue to show signs of macrophage/microglia activation and intrathecal immunoglobulin production in the CNS.