The authors have made electroencephalographs studies in cases of cerebral abscess since 1951; these records were always obtained before any neurosurgical treatment and were repeated several times after the surgical approach. The authors call the attention to the important data that the EEG can offer to the diagnosis of cerebral abscess and to the prognosis in patients who were operated on. Twenty-two patients (20 had EEG previously to the surgical treatment, 17 with repeated EEG after the operation and 2 had EEG only after the operation) were clinically and electroencephalographically studied, the diagnosis being confirmed by arteriograms and by the surgical treatment. AH patients were operated on and received antibiotics. In the present paper the EEG changes found during the first month following the surgical treatment were not included. In the group of patients with EEG obtained before the surgical treatment, the authors found difuse changes in the record of the 20 patients (100 per cent), and focal changes consisting mainly by delta waves of 0.5-3 c/s, potential of 400-500 mV (19 cases) focal depression of the basic rhythm (11 cases); epileptogenic focus was found only in 2 cases. The very slow delta waves and focal depression of the basic rhythm represent very important data for the diagnosis, particularly when studied in relation to the clinic data. In the group of patients who had clinical and EEG examinations after the operation, being the EEG records repeated during a period of time from one to 60 months, the authors found signs of active convulsive focus in 10 out of 17 patients. In the opinion of the authors this incidence is very high and it is in accordance with the high frequency of epilepsy following cerebral abscess. The lenght of time between the operation and the onset of the focal activity varied from one to 25 months. Out of the 10 patients with focal activity and with clinical epilepsy, only 2 patients had presented focal activity before to the "healing" of the abscess. The authors discuss this problem and believe that the healing, which is a chronic process, is the responsible for the onset of these clinically and electroencephalographically manifestations. In the same group only 4 patients had normal EEG during all the follow-up; 2 patients had normal EEG during 5 and 15 months, but developed focal activity later on. Other 3 patients had signs of suffering of the nervous tissue (delta waves) from one to 3 months after the operation (patients with convulsive seizures or still with signs of abscess in evolution).. In 6 cases there was slight changes of the basic rhythm, which were due to the healing and with no clinical correlation. The authors conclude that the EEG is a very important auxiliary method for the diagnosis of cerebral abscess and for the prognosis of those cases surgically treated, since it shows disappearance of acute signs of cerebral involvement, or early formation of active convulsive focus, permitting the begining of the anticonvulsive therapy.