Nosocomial infections at a department of neurosurgery were followed prospectively. In 1999, a high incidence of nosocomial lower respiratory tract infections (NLRTI) was observed in patients after intracranial artery aneurysm surgery (ICAAS). From February to December 2000, a short course of ciprofloxacin and co-trimoxazole was given prophylactically to all patients with ICAAS. The incidence of nosocomial infections in patients after ICAAS fell from 78.4 to 30.9% (P<0.0001). The incidence of NLRTI fell from 43.3 to 13.6% (P<0.0001) and the incidence of urinary tract infections from 12.4 to 2.5% (P=0.015). No significant change in antibiotic sensitivity at the department was observed. Antibiotic use decreased from 100.4 defined daily doses (DDD) to 85.4 DDD per 100 bed-days.