The clinical and pharmacokinetic risk factors for toxicity after high-dose methotrexate (MTX) in children with acute lymphoblastic leukemia were evaluated using a multivariate statistical analysis. Plasma samples were collected after 44 24-h infusions of MTX (5 or 8 g/m2) in 13 children (age 3.3-12.9 years) and subsequently analyzed by HPLC to determine the MTX and 7-hydroxymethotrexate (7-OHMTX) concentrations. Toxicity was evaluated according to the WHO criteria. Severe toxicity was not observed. Oral mucositis (WHO grade > or = 1) was significantly related to a high plasma MTX concentration at 28 h after starting the infusion (p = 0.013), a low ratio of plasma 7-OHMTX/MTX at 66 h after starting the infusion (p = 0.049), and a slow clearance of MTX (p = 0.048). The risk of leukopenia (WHO grade > or = 2) increased significantly with the number of courses (p = 0.02). Increasing age and a long exposure to a high MTX concentration in plasma (AUC) were significant risk factors (p = 0.047 and p = 0.009, respectively) for developing elevated liver enzymes (ALAT) (WHO grade > or = 2). This study shows how a statistical model can be used to identify clinical and pharmacokinetic factors that may influence MTX-induced toxicity. The therapeutic ratio could thereby potentially be improved.