Pyogenic infections of the brain are divided according to the anatomic structures they involve. Brain abscess and subdural empyema are the main phenomena. This retrospective study analyses case records of the Neurosurgical University Teaching Hospital at Münster from a bacteriological standpoint and compares them with other reports in the literature. Various kinds of bacteria depending on etiology and pathogenesis are described. It is found that with improved bacteriological techniques the number of anaerobic infections increases and, at the same time, the number of sterile empyema decreases. A significantly higher mortality of patients with sterile suppuration is also found. It must be assumed that insufficient bacteriological evaluation leads to faulty diagnoses and ineffective antibiotic therapy and that genuine sterile empyema is rare.