Treatment of maternal chronic myeloid leukemia with imatinib mesylate is avoided because of potential fetal effects. Two women with progression of disease during pregnancy required imatinib therapy. Concentrations of imatinib in maternal blood, placenta, umbilical cord blood and breast milk were 886, 2452, 0 to 157, and 596 ng/ml, respectively. Concentrations of the active metabolite CGP74588 in maternal blood, placenta, umbilical cord blood and breast milk were 338, 1462, 0 and 1513 ng/ml, respectively. As Imatinib and CGP74588 cross the mature placenta poorly, use of the drug after the first trimester may be reasonable under some circumstances. Imatinib and CGP74588 are found in breast milk, and therefore avoidance of breastfeeding is advisable.