Bone marrow biopsy findings in representative HES patients. Biopsy sections from patients with HES with (A-C) and without (D-F) elevated serum tryptase levels were stained with hematoxylin and eosin (A,D), antitryptase antibody (B,E), and antireticulin antibody (C,F). Although the bone marrow biopsy specimens from both patients show a dramatic increase in eosinophils and myeloid precursors (A,D), only the specimen from the patient with elevated serum tryptase shows marked hypercellularity (A), atypical spindle-shaped mast cells (B), and reticulin fibrosis (C).