303 patients underwent brain biopsy; 20 patients were available for a follow-up EEG examination, up to 16 years after the biopsy. 4 patients (20%) had focal fits starting within 6 months after the intervention. In the EEG of 15 patients (75%) 2 varieties of focal abnormalities appeared: "trepanation activity" which, for reasons discussed below should be regarded as an abnormality; and other focal features, including spikes, more often seen in patients with epilepsy. It is concluded that irritative brain lesions appear in a considerable number of patients after brain biopsy. The decision of performing this diagnostic procedure should be made after taking in consideration that: by enhancing the possibility of a correct diagnosis we may induce focal seizures in every fifth patient and irritative phenomena in three fourths of their EEG'S.