Deletions of derivative chromosome 9 [der(9)] can be identified by fluorescence in situ hybridization (FISH) in 10% to 15% of patients with chronic myeloid leukemia (CML). Patients with der(9) deletions have been reported to have an adverse outcome when treated with chemotherapy, interferon, and possibly imatinib mesylate. We investigated the frequency and prognostic significance of der(9) deletions among 352 patients with CML treated with imatinib mesylate at our institution, in whom a deletion status of der(9) was determined. Thirty-three patients (9%; 95% CI 0.07, 0.13) (30 in chronic phase, 3 in accelerated phase) had der(9) deletions. The rates of major (82% vs 79%, P = 0.82) and complete cytogenetic response (76% vs 66%, P = .33) with imatinib mesylate therapy were similar in patients with and without der(9) deletions, respectively. After a median follow-up of 28 months, there was no difference in overall survival (P = .30) or response duration (P = .49) in patients with and without deletions. In a multivariate analysis, der(9) deletions had no significant impact on response, survival, or response duration. We conclude that treatment with imatinib mesylate overcomes the adverse prognostic significance of der(9) deletions in patients with CML.