Toxin-producing Clostridium difficile is the commonest bacterial cause of nosocomial diarrhoea and is a well recognized cause of hospital outbreaks in elderly care units. High C. difficile disease rates have been associated with the use of broad-spectrum antibiotics, especially cephalosporins. An outbreak of C. difficile infection in the elderly care unit at Gloucestershire Royal NHS Trust continued despite increased ward cleaning and strict implementation of infection control measures. A restrictive antibiotic policy that would maintain colonization resistance in the gastrointestinal tract was introduced throughout this unit. Patients admitted with suspected infection were prescribed intravenous (i.v.) benzylpenicillin 1.2-1.8 g every 6 h to cover streptococcal infections and i.v. trimethoprim 200 mg twice daily to cover urinary tract pathogens and Haemophilus influenzae. If the patient had septic shock a single iv dose of gentamicin was given (120-180 mg) to cover more resistant gram-negative bacilli. The following were monitored before and after the policy change. The number of cases of C. difficile toxin-positive diarrhoea; cefuroxime and total antibiotic use on the elderly care wards; patient mortality rates; and length of hospital stay: two hundred and fifty-two and 234 patients respectively with a discharge diagnosis of infection were admitted before and after the antibiotic policy change. Mortality rates and length of hospital stay were unchanged. Cefuroxime prescribing and total antibiotic prescribing costs fell by 5150 pounds sterling and 8622 pounds sterling respectively in the 7 month period after the change. Thirty-seven cases of C. difficile diarrhoea occurred in the period before and 16 in the period after the policy change. The incidence of C. difficile diarrhoea and of cefuroxime use has remained low since then. The use of narrow-spectrum antibiotics for hospital treatment of community-acquired infections in the elderly should be encouraged. Outbreaks of C. difficile diarrhoea should be managed with the combined approach of infection control and strict antibiotic policies.