43 cerebrospinal fluid (CSF) specimens obtained from 20 children with tuberculous meningitis (TBM) at varying times during the first month of treatment were examined by polymerase chain reaction (PCR) for the presence of M. tuberculosis DNA. Overall 27 CSF specimens (63%) from 16 patients (80%) gave>> or = 1 positive results and positive results were obtained from CSF specimens throughout the first 4 weeks of therapy. Nine CSF specimens (21%) gave a doubtful result (only 1 of duplicate determinations positive) and 7 (16%) a negative result. CSF from patients with suspected TBM should be submitted for PCR evaluation and positive results may be obtained up to at least 4 weeks after the start of treatment.