PURPOSE: The Prospective Oral Mucositis Audit assessed the incidence, duration, and determinants of severe oral mucositis (OM; WHO oral toxicity scale grades 3 to 4) in patients with multiple myeloma (MM) or non-Hodgkin's lymphoma (NHL) receiving high-dose conditioning chemotherapy before autologous stem-cell transplantation. PATIENTS AND METHODS: Patients with MM (n = 109; mean age, 57 +/- 8 years) or NHL (n = 88; mean age, 50 +/- 13 years) were treated with high-dose melphalan (200 mg/m(2)) or carmustine 300 mg/m(2), etoposide 800 mg/m(2), cytarabine 800 to 1,600 mg/m(2), and melphalan 140 mg/m(2) chemotherapy, respectively, in 25 European centers. OM assessments were made daily until 30 days after transplantation or hospital discharge. High quality of OM assessment was ensured by an intensive training program. RESULTS: Severe OM occurred in 46% (95% CI, 36% to 56%) of patients with MM and 42% (95% CI, 32% to 53%) of patients with NHL, with a mean duration of 5.3 days (95% CI, 4.4 to 6.1 days) and 5.5 days (95% CI, 4.5 to 6.7 days), respectively. Time from start of conditioning to peak OM score was 12.1 +/- 2.6 and 14.6 +/- 2.4 days. Severe OM risk and/or duration was significantly associated with higher chemotherapy dose per kilogram of body weight and poor performance status, but in contrast with some previous reports, this was not related to age. CONCLUSION: Severe OM is more common in the transplantation setting than previously reported, justifying effective preventative and therapeutic measures.