Mouth ulcers are a frequent cause of morbidity in patients rendered neutropenic as a result of chemotherapy. We report here a series of 28 such patients from whom swabs were taken for viral isolation and mycological culture. In 13 patients, herpes simplex virus (type I) was isolated and in 17 patients Candida albicans was cultured. Both organisms were isolated in 9 patients. Our results suggest that both a viral and fungal element may be important in the aetiology of oral ulceration and that antiviral and antifungal agents may each have a role in the prophylaxis and treatment of such patients.