A 49-year-old man presented with an 8.8 × 6.5 cm, left infraclavicular subpectoral mass. A biopsy showed predominantly spindle-ish cells (panels A-B, hematoxylin and eosin stain; original magnifications shown on pictures) and was therefore worked up for soft tissue tumors initially. The spindle-ish cells were strongly positive for CD31 with membranous and cytoplasmic staining (panel C) but were negative for CD34, AE1/AE3, smooth muscle actin, desmin, and S100 (not shown). Further workup revealed positive stains for CD45 (panel D), CD20 (panel E), Bcl-2 (panel F), Mum-1 (panel G), and C-myc (panel H); partially positive stains for Bcl-6 (panel I); weakly positive stains for Pax-5 (not shown); and high Ki-67 proliferation index (90%, not shown). The cells were negative for CD3, CD10, LMP1, P53, CD5, and Bcl-1 (not shown). The overall findings are most consistent with a diffuse large B-cell lymphoma (DLBCL), activated B-cell subtype, with spindle-ish cell morphology and strong aberrant expression of CD31.A few previous reports indicated that CD31 was detected in small cell lymphomas (by flow cytometry), plasmablastic lymphoma (weak stain), and cutaneous spindle cell B-cell lymphoma (focal weak stain). We believe this is the first documented case of DLBCL with strong diffuse expression of CD31. The spindle-ish cell morphology and strong CD31 expression create diagnostic difficulties.