Neurosurgical patients with post-operative meningitis often present with negative bacterial cultures. The symptoms and signs as well as laboratory findings are identical to those with verified bacterial meningitis. The aim of this study was to find out whether we are dealing with a sterile reaction, and antimicrobial treatment can safely be stopped. 24 patients with post-operative meningitis with negative bacterial cultures were randomized into two groups. Both were initially treated with chloramphenicol until the results of the bacterial cultures were available. Treatment was then withdrawn in one group and continued in the other. Chloramphenicol had no effect on the outcome and can therefore safely be stopped when adequate bacterial cultures are reported negative.