Oral mucosal ulceration complicating bone marrow transplantation interferes with patients' comfort, nutrition and may lead to systemic infection derived from the mouth. The mucosal injury results from epithelial damage due to the cytotoxic effects of chemotherapy and radiation conditioning as well as from superficial oropharyngeal infection. Because chlorhexidine gluconate is a broad spectrum topical antimicrobial which has been demonstrably effective in preventing oral infection and gingivitis, we performed a randomized, placebo controlled, double-blind trial of chlorhexidine as a mouth rinse in BMT recipients to study the severity of oral mucositis and both oral and systemic infectious complications. One hundred patients were randomly assigned to receive either chlorhexidine gluconate 0.12% mouth rinse or placebo three times daily from the initiation (day -8) of chemoradiotherapy conditioning until day +35 post-BMT. Chlorhexidine use resulted in a trend toward improved oral hygiene index (reduced dental plaque) (p = 0.06) but did not modify the oral mucositis. Patients using chlorhexidine developed a maximum ulceration of 18 +/- 22% of their oral mucosa, while placebo patients ulcerated 25 +/- 31% of the mouth. Ulcerative mucositis was significantly worse in adults compared with children, in individuals who received methotrexate for graft-versus-host disease prophylaxis, and was most prominent on non-keratinized epithelium. Overall, there was no clinically demonstrable additional therapeutic advantage to the use of chlorhexidine in either reducing the mucositis, controlling oral pain, facilitating oral nutrition, shortening hospital stay, or reducing oral infection with herpes simplex virus. There was a trend toward diminished oral candidiasis in chlorhexidine users (p = 0.06).(ABSTRACT TRUNCATED AT 250 WORDS)