Acute myeloid leukemia (AML) with inv(16)(p13q22), also known as M4Eo, is a distinct type of AML with a favorable prognosis associated with abnormal bone marrow eosinophils. We reviewed the morphologic findings of archival bone marrow specimens with M4Eo, specifically assessing for dysplasia, and performed immunohistochemical studies to assess the growth fraction using the MIB-1 (Ki-67) antibody. We also assessed the apoptotic rate by terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate-nick end labeling. All assessable cases had more than 10% dysplastic forms in at least 1 lineage. Seventeen cases had 10% or more dysplastic forms, and 3 cases had more than 50% dysplastic forms in at least 2 lineages. Immunoreactivity for Ki-67 was higher in M4Eo than in other AML types (P = .000). The apoptotic rate in M4Eo was similar to other AML types (P = .724). Our data show that dysplasia is a prominent feature, but not a prognostic indicator, in M4Eo. M4Eo is associated with a significantly higher proliferation rate than other AML types.