Vincristine is a key drug in the treatment of childhood and adult acute lymphoblastic leukemia (ALL), and many other childhood malignancies. Despite decades of wide clinical use, no data on the correlation between vincristine pharmacokinetics and long-term clinical outcome have been published. We here report clinical data (median follow-up time 10.5 years, range 7.3-12 years) for 86 children with B-cell precursor ALL, in whom vincristine kinetics were studied on treatment day 1. The median total plasma clearance was 429 and 331 ml/min per m(2) and the area under the plasma concentration-time curve (AUC) was 4.49 and 5.40 mg/l x min in relapse and non-relapse patients, respectively (not significant). In standard risk patients, where treatment depends more heavily on vincristine than in other subgroups, the relative risk (RR) of relapse was significantly increased for patients with clearance values above median (RR 5.2; P = 0.036), or AUC values below median (RR 5.8; P = 0.025). Our data suggest a relationship between the antileukemic effect and the systemic exposure of the drug, which warrants further studies.