Relapse occurs frequently after allogeneic hematopoietic cell transplantation (HCT) for treatment of high-risk Philadelphia chromosome-positive (Ph+) leukemia. Administration of imatinib early after HCT might provide an effective approach for preventing recurrent Ph+ leukemia, but the feasibility of this approach has not been systematically tested. Twenty-two patients, 15 with Ph+ acute lymphoblastic leukemia and 7 with high-risk chronic myelogenous leukemia, were enrolled in a prospective study and given imatinib from the time of engraftment until 365 days after HCT. Before day 90, adults (n = 19) tolerated a median average daily imatinib dose of 400 mg/d (range, 200-500 mg/d), and children (n = 3) tolerated 265 mg/m2/d (range, 200-290 mg/m2/ d). The most common adverse events related to imatinib administration were grade 1-3 nausea, emesis, and serum transaminase elevations. We conclude that imatinib can be safely administered early after myeloablative allogeneic HCT at a dose intensity comparable to that used in primary therapy.