A single-patient randomised clinical trial was used to determine optimum treatment for a patient with inflammation of her continent ileostomy. The trial proved to be feasible and acceptable to both patient and clinician. As a result, both the diagnosis of inflammation and the efficacy of metronidazole in treating it were confirmed. The trial design, in which multiple courses of active and control treatments are given to one patient, may be useful in other clinical situations to determine the most effective therapy in an individual case.