Twenty-eight adults with chronic myelogenous leukemia (CML) that had relapsed after allogeneic stem cell transplantation (SCT) received imatinib mesylate (400-1000 mg/d). Disease was in chronic phase in 5 patients, accelerated in 15, and blastic in 8 (7 medullary, 1 extramedullary); median time from transplantation to relapse was 9 months (range, 1-137 months). Thirteen patients had undergone salvage donor lymphocyte infusion (DLI) (median time from DLI to imatinib mesylate therapy, 4 months [range, 2-39 months]). The overall response rate was 79% (22 of 28 patients); the complete hematologic response (CHR) rate was 74% (17 of 23 patients), and the cytogenetic response rate was 58% (15 of 26 patients; complete response in 9 [35%] patients). CHR rates were 100% for chronic phase, 83% for accelerated phase, and 43% for blastic phase. The patient with extramedullary blastic disease achieved complete response. Cytogenetic response rates were 63% (12 of 19 patients) for chronic or accelerated phases (complete cytogenetic response in 8) and 43% for blastic phase (3 of 7 patients). At median follow-up of 15 months, 19 patients were alive, 9 with no evidence of disease. The 1-year estimated survival rate was 74%. Five patients had recurrence of grade 3 (3 patients) or grades 1 to 2 (2 patients) graft-versus-host disease (GVHD). Severe granulocytopenia developed in 43% of patients and thrombocytopenia in 27%; both conditions reversed with dose adjustments of imatinib mesylate. We conclude that imatinib mesylate effectively controlled CML that recurred after allogeneic SCT, but it was associated with side effects including myelosuppression and recurrence of severe GVHD.