A 6-year-old girl with chronic myeloid leukemia (CML) was treated with imatinib 230 mg/m2/day and its pharmacokinetics were investigated. The patient had a complete hematologic response on day 21, but had a minor cytogenetic response and the CML progressed to a blast crisis on day 133. At present, she has maintained complete cytogenetic remission with allogenetic peripheral blood stem cell transplantation. The pharmacokinetics revealed that the maximum concentration (1.4 micrograms/ml); time to maximum concentration (5.1 h); half-life (11.0 h); trough concentration (0.4 microgram/ml); and, area under the concentration-time curve (28.1 micrograms.h/ml) were inferior to those for adult patients in the 400 mg/day group. This observation suggests that a suboptimal plasma concentration might be related to resistance to imatinib and/or blast crisis.