The polymerase chain reaction (PCR) was used to detect Mycobacterium tuberculosis DNA in 29 CSF specimens from seven patients with tuberculous meningitis after the start of antituberculous chemotherapy. Ten of the 13 CSF specimens taken from these patients with an initial treatment of three weeks were positive for the PCR study. By contrast, only one of the other 16 CSF specimens taken from patients treated for more than three weeks was positive. This study shows that M tuberculosis DNA can exist in the CSF of a patient with tuberculous meningitis for three weeks after treatment and that PCR can still be a sensitive method to detect M tuberculosis DNA in the CSF after the start of treatment in patients with tuberculous meningitis.