The Prospective Oral Mucositis Audit was an observational study in 197 patients with multiple myeloma (MM) or non-Hodgkin's lymphoma (NHL) undergoing, respectively, high-dose melphalan or BEAM chemotherapy and autologous SCT at 25 European centres. We evaluated the relationship between severe oral mucositis (SOM; WHO Oral Toxicity Scale grade 3-4) and local and systemic clinical sequelae and medical resource use. SOM occurred in 44% of patients. The duration of SOM (mean 5.3 days) correlated with time to neutrophil engraftment. The following parameters increased gradiently with maximum grade of oral mucositis: duration of pain score X4, opioid use, dysphagia score X4, total parenteral nutrition (TPN) use, incidence and/or duration of fever and infection, and duration of antibiotic use. SOM increased the duration of TPN use by 2.7 days (Po0.001), opioids by 4.6 days (Po0.001), and antibiotics by 2.4 days (P ¼ 0.045). SOM prolonged hospital stay by 2.3 days (P ¼ 0.013) in MM patients, but not in NHL patients (who tended to have a longer hospital stay).In conclusion, this analysis of prospectively collected observational data provides important insight into the scope and impact of SOM in the European transplant setting.