Suggested diagnostic studies in FL grade 3. Upper left: Cells from FL grade 3A are shown with hematoxylin and eosin (H&E) and Giemsa stains. Note the admixture of centrocytes and centroblasts (>15 per high power field) highlighted in the Giemsa stain. Upper right: Cells from FL grade 3B are shown with H&E and Giemsa stains. The follicles are composed of sheets of centroblasts with open chromatin, several nucleoli, and abundant basophilic cytoplasm highlighted with the Giemsa stain. Upper middle: Cells from FL with ambiguous morphology are shown. They are medium-size with open chromatin but inconspicuous nucleoli unlike centroblasts (arrows) and without the cytologic features of centrocytes. With ambiguous morphology (blue arrow), the presence of BCL2 rearrangement and/or CD10 expression favors the diagnosis of FL grade 3A; if both are absent, a diagnosis of FL grade 3B is favored. In patients who have FL grade 3B with IRF4/MUM1 expression, IRF4-FISH analysis is recommended to exclude the diagnosis of large B-cell lymphoma with IRF4 rearrangement. Original magnification ×400. BM, bone marrow.