Clostridium difficile infection has become endemic in many hospitals and yet few data on the associated costs of such cases are available. We prospectively followed 50 consecutive cases of C. difficile infection and 92 control patients, who were admitted to the same geriatric wards within 72 h of the cases. Cases and controls had similar age, sex and major diagnosis distributions. Cases stayed significantly longer (mean 21.3 days, median 20.5 days; P < 0.001) in hospital than controls, including an average 14 days in a side room. Diarrhoea developed in cases on average 10.8 days after admission, which, when compared with a mean duration of stay for controls of 25.2 days, implies that C. difficile infection caused an increased duration of stay, as opposed to infection occurring because of longer residence. There was a significantly higher death rate in cases compared with controls (P < 0.01). Antibiotic treatment of C. difficile infection cost an average of Pounds 47 per case. The average number of laboratory investigations per day was similar for cases and controls, but the increased length of stay meant an extra cost for tests of approximately Pounds 210 per case. Assuming hotel costs of Pounds 150 (Pounds 200) per day stay (in a side room), 94% of the additional costs associated with C. difficile infection were due to increased duration of stay (Pounds 3850). The total identifiable increased cost of C. difficile infection was, therefore, in excess of Pounds 4000 per case. Such high costs can be used to justify expenditure on personnel and/or other control measures to reduce the incidence of this hospital-acquired infection.