Sera and CSF from 29 patients in early and late stages of HIV infection were analysed for intrathecal antibody production. Elevated CSF-IgG indices indicating intrathecal IgG synthesis were demonstrated in 9 patients while 4 of 18 patients tested had oligoclonal IgG bands in the CSF. Analysis of HIV-specific antibodies by enzyme-linked immunosorbent assay (whole antigen and site-directed ELISA) and calculation of "antibody indices" (CSF/serum antibody quotient divided by CSF/serum albumin quotient) indicated intrathecal HIV antibody synthesis in 19 patients. Analysis of serum and CSF antibodies by an imprint immunofixation (IIF) method showed intrathecal synthesis of predominantly polyclonal HIV-IgG antibodies in 11 of 13 patients examined. IIF analysis of antibodies to six other infectious agents showed no intrathecal antibody production except in one patient who had minor fractions of intrathecally synthesized IgG antibodies to varicella zoster virus. The present results demonstrate that an intrathecal HIV-specific antibody response may be present in both early and late stages of HIV infection, and indicates that HIV may reach the brain at an early stage of infection.