Six HIV-antigenaemic patients with AIDS or AIDS-related complex were studied to assess the effect of treatment with low-dose zidovudine (250 mg) in 6-hourly doses on HIV antigen (HIV-Ag) levels in cerebrospinal fluid (CSF). HIV-Ag was detected in CSF of three patients before treatment. These patients became CSF HIV-Ag-negative within 8 weeks of treatment. One initially CSF HIV-Ag-negative patient became strongly CSF HIV-Ag-positive during interruption of zidovudine treatment; CSF HIV-Ag disappeared again after treatment was restarted. None of our patients showed a significant neurological improvement during the study. These results show that low-dose zidovudine can suppress viral expression in CSF. Whether suppression of viral replication can prevent future HIV-related neurological disease remains to be investigated.