Mantle cell lymphoma involving spleen in a 60 y/o male. He presented with massive splenomegaly and underwent splenectomy. The spleen measured 25 x 17 x 6.5 cm and weighed 2450 grams. The cut surface showed multiple, gray-tan nodules ranging in size from 0.1 cm to 0.5 cm. Microscopic examination showed markedly expanded white pulp consisting of a monomorphic population of small to medium sized lymphocytes. The cells have slightly irregular nuclei with folds and indentations, condensed chromatin, and scant cytoplasm. The background contains scattered histiocytes. Mitotic activity is increased. Immunophenotyping by flow cytometry showed the tumor cells to be positive for CD19 and CD20 and negative for CD5 and CD10 with light chain restriction (lambda). There was strong immunoreactivity for CD20 and Cyclin-D1. FISH analysis showed cyclin-D1-IgH gene fusion.