The limit of detection of Mycobacterium tuberculosis in spiked cerebrospinal fluid (CSF) using polymerase chain reaction (PCR) was compared to that of a radiometric liquid culture. Serial dilutions of clinical isolates of Mycobacterium tuberculosis were prepared in CSF (n=3) or broth (n=11) with estimated concentrations of 0-550 cfu/ml. Each dilution was examined concurrently by PCR and radiometric culture. PCR and radiometric culture detected Mycobacterium tuberculosis DNA in all dilutions with an estimated 2 cfu/ml in the CSF. At lower concentrations (estimated <2 cfu/ml), PCR and radiometric culture were positive in three of five (60%) and five of five (100%) CSF samples, respectively. In comparison to PCR in broth dilutions, no evidence of inhibition or interference was noted. These results imply that PCR can provide a rapid and reliable diagnosis of tuberculous meningitis, although there is a potential for false-negative results to occur in samples containing very few organisms ( < 2 cfu/ml).