Data concerning to 840 patients submitted to neurosurgical operations during 5 years (1962-1967) were reviewed in order to verify the occurrence of acute inflammatory leptomeningeal reactions (RIL) in the immediate post -operative period. It was verified that RIL occurred in 106 cases (12,6%). In one case the demonstration was made at the necroscopic examination; in 105 cases the demonstration was based in the cerebrospinal fluid (CSF) examination. Clinical and CSF data concerning to the last 105 cases were studied. It was shown that fever, headache, meningism and changes in consciousness were the symptoms that most commonly suggested the diagnosis. Symptoms were more conspicuous about the tenth day (mean) of the post-operative period. Considering the CSF changes found, the cases were divided in three groups: probably aseptic RIL (72 cases), probably septic RIL (26 cases) and septic RIL (7 cases). Only in the last group the bacteriological examination of the CSF sample was positive. In 59 cases there occurred the association of RIL and sub-arachnoid haemorrhage. The severity of this association was pointed out based upon the incidence of deaths among the patients studied. Twenty four patients have died; in 20 the RIL and subarachnoid haemorrhage association has occurred.