HCL involving blood, bone marrow, and brain. (A) Hairy cells in the peripheral blood showing oval to slightly irregular nuclei, ground-glass chromatin, and abundant pale blue cytoplasm with hairy projections (Wright-Giemsa stain, oil immersion lens, original magnification ×1000). (B) Bone marrow shows extensive interstitial infiltration by HCL (hematoxylin and eosin [H&E] stain, original magnification ×200). (C) Brain biopsy showing fragments of brain parenchyma (right) and area of atypical lymphoid cells (HCL infiltrate) with focal area of necrosis (left) (H&E stain, original magnification ×50). (D) Atypical lymphoid cells involving the brain are small, with slightly irregular nuclei and inconspicuous nucleoli; scattered hemosiderin-laden macrophages are admixed (H&E stain, original magnification ×400). The HCL infiltrate is positive for (E) CD20 and (F) BCL1 (immunohistochemical stains, original magnification ×200).