Cyclic adenosine monophosphate (cAMP) content of CSF in 94 cases of nervous system inflammatory diseases was measured. Statistically significant increases were found in tuberculous meningitis, encephalitis, cerebral arachnoiditis and cysticercosis cerebralis showing the most prominent increase, varying in accordance with the severity and progress of the disease. The mechanism of increase in CSF cAMP is briefly discussed in relation to the literature and the possible clinical application of CSF cAMP content measurement.