The aim of this work was to study the usefulness of CSF adenosine deaminase determination in the diagnosis of tuberculous meningitis and determine if the proposed cutoff value of 7.1 i.u./ml had the better sensitivity and specificity. We retrospectively studied 148 patients, 12 with tuberculous meningitis and 136 with other central nervous system diseases. Adenosine deaminase values ranged from 3.6 to 31.2 i.u./ml in patients with tuberculous meningitis and from 0.1 to 312 i.u./ml in controls. The best sensitivity/specificity ratio (83.3 and 85.3% respectively) was obtained using a cutoff value of 6.5 i.u./ml. It is concluded that CSF adenosine deaminase values are useful in the diagnosis of tuberculous meningitis and that the cutoff value should be lowered to 6.5 i.u./ml to improve its diagnostic yield.