Oral mucositis is a dose-limiting toxicity of intensive chemotherapy. It is caused directly by the cytotoxic effect of chemotherapeutic agents and indirectly by sustained neutropenia. Severe oral mucositis is an important predisposing factor for life-threatening septic complications during aplasia. It also reduces quality of life. At present, no effective causal prophylaxis or treatment against oral mucositis is established. We performed a prospective randomised placebo-controlled trial using topical oral r-metHuG-CSF (filgrastim) in high-grade lymphoma patients treated according to the B-NHL protocol, which contains high-dose methotrexate and causes severe oral mucositis (WHO grades I-IV) in >50% of patients. Between August 1996 and July 1997, a total of 32 chemotherapy cycles were documented in eight patients (four male, four female). Mucosal erythema and ulceration were recorded. All patients assessed their oral pain and impact on swallowing daily, using a subjective scale from no to maximal discomfort (1-10). In addition, oral mucositis was assessed according to the WHO score. Filgrastim was administered in 16 cycles as a viscous mouthrinse (carboxymethylcellulose 2%, oleum citrii) 4 x 120 microg/day from days 10 to 16. Sixteen cycles were given to control patients, of these 14 with placebo, and another two cycles with no treatment. Severe mucositis (WHO grade III/IV) was documented in 21 of 32 cycles (65.5%). A difference of borderline significance was observed for the reduction of maximum severity of oral mucositis between G-CSF vs placebo (P = 0.058), with a reduction of WHO grade IV of 50% (four G-CSF vs eight control). The number of days in hospital was reduced significantly in the G-CSF group (P = 0.02). In conclusion, topical oral G-CSF mouthrinses may be beneficial to reduce oral mucositis.