We report the case of a 78-year-old man in whom routine physical examination revealed cervical adenopathy and splenomegaly. Peripheral blood showed a normal white blood cell count with an absolute lymphocytosis, which included a population with slightly indented nuclei. Lymph node biopsy showed morphology compatible with mantle cell lymphoma. Bone marrow biopsy showed replacement by a lymphoid proliferation composed of lymphocytes with features similar to those found in the peripheral blood. Immunophenotypic analysis of both peripheral blood and lymph node showed positivity for CD19, CD20 and CD22, with lambda light chain restriction. Tests for CD5 and CD10 were negative. Cytogenetic analysis and polymerase chain reaction studies confirmed the presence of t(11,14) supporting a diagnosis of mantle cell lymphoma. This unusual case of CD5-negative mantle cell lymphoma exemplifies the importance of combined molecular, cytogenetic, and morphologic evaluation when confronted with a lymphoma having an atypical phenotype.