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 >or=4, opioid use, dysphagia score >or=4, 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 (P<0.001), opioids by 4.6 days (P<0.001), and antibiotics by 2.4 days (P=0.045). SOM prolonged hospital stay by 2.3 days (P=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.