The post-operative infection-rate in neurosurgical patients who received prophylactic antibiotics was compared to the infection-rate of patients who received no antibiotics. None of the 73 studied patients had pre-operative infection. Infections occured in 26,4% of the patients in the first group and in none of the second group (p < 0,2); 27,5% of the patients with intracranial lesions and 9% of the patients with spinal lesions in the first group had post-operative infections, and none in the second group (p < 0,05 and 0.05 < p < 0.1). 67% of all the patients had severe neurological lesions; 27% of these had post-operative infections; only 4% of the patients with mild lesions had infections (p < 0.05). Post-operative infections were severe and fatal in most of these cases. Prophylactic broad-spectrum antibiotics were of no value in preventing post-operative infections in these neurosurgical patients and those who received no antibiotics had a significantly lower rate of infections.