Oral mucositis is a common problem after high-dose methotrexate (HD-MTX) treatment. Our purpose was to identify factors associated with the development of mucositis in children with ALL who had no delayed elimination of methotrexate (MTX) according to the conventional criteria (p-MTX at 42 hours < 1 mumol/L and at 66 hours < 0.2 mumol/L). Pharmacokinetic studies of MTX and the metabolite 7-hydroxymethotrexate (7-OHMTX) in plasma and saliva were carried out in 13 children treated with HD-MTX (5-8 g/m2 intravenously over 24 hours for a total of 44 courses). Oral mucositis was evaluated according to the criteria of the World Health Organization. Mucositis was observed in 52% of the infusions. In 28 infusions with no delayed elimination of MTX 39% developed mucositis, which was found to correlate significantly with low systemic clearance of MTX during the infusion and a low p-7-OHMTX/p-MTX ratio at 66 hours after the start of infusion. The MTX concentration in saliva did not show any correlation with the development of mucositis. The present conventional criteria for high-risk MTX concentrations might need to be reevaluated because a high percentage of patients still suffer from oral toxicity despite "normal" elimination. A reduced ratio between the simultaneous concentrations of 7-OHMTX and MTX in plasma may be a possible mechanism of this "unpredictable" oral toxicity.