One hundred and ten patients with Philadelphia chromosome (Ph)-positive chronic phase chronic myelogenous leukemia (CML) post-interferon-a failure treated with imatinib mesylate therapy were analyzed for the prognostic significance of marrow reticulin stain-measured fibrosis. The median time from diagnosis was 31 months. Severe reticulin (grade 3 - 4) fibrosis was observed in 67 patients (61%). Patients with severe marrow fibrosis had similar complete cytogenetic response rates with imatinib (67 vs. 58%; P = 0.45) compared with those with mild?-?moderate fibrosis. The estimated 4 year survival rates (80 vs. 88%; P = 0.27) and failure-free survival rates (69 vs. 77%; P = 0.34) were also not different. We conclude that the previously established poor prognostic significance of marrow fibrosis in CML is less relevant with imatinib therapy.