A system was developed for scoring oral mucositis in order to investigate its connection with fever and bacteraemia due to 'viridans' streptococci. A series of 42 allogeneic bone marrow transplant recipients given demethoxydaunorubicin and total body irradiation for conditioning therapy were monitored daily for the presence of lesions, erythema, oral oedema, pain and dysphagia, each of which was graded numerically at four levels. These values were added together to yield a daily mucositis score (DMS) with a scale of 0-15. Mucositis developed shortly after transplant and progressed within a few days to grade III (WHO grade 3-4) in the majority of patients. Bacteraemia due to 'viridans' streptococci was documented in 64% of cases and the organisms were first detected as fever developed and mucositis approached its peak. The WHO scheme defined mucositis as either absent or grade 3-4 corresponding to a DMS of 4 or 5, whereas a grading system based on the most pronounced sign or symptom resulted in three grades of severity which corresponded to a DMS of less than or equal to 4, 5-9 and greater than or equal to 10 respectively. However, only the DMS permitted monitoring of mucositis through all its stages of development. The scheme therefore offers the potential for exploring causal relationships between mucosal damage, granulocytopenia, fever and bacteraemia and can be included as an independent measure in studies of prevention and therapy of complications related to mucositis.