Mantle cell lymphoma (MCL) is a non-Hodgkin lymphoma with a poor prognosis that may be confused with less aggressive diseases, such as small lymphocytic lymphoma and follicular lymphoma. In many cases immunophenotyping, particularly analysis of reactivity for CD5 and CD10, is an important adjunct to morphology that usually distinguishes MCL from follicular lymphoma; the former is CD5(+)/CD10(-), whereas follicular lymphoma is the reverse. We report a case of MCL, initially diagnosed as follicular lymphoma, that at presentation expressed neither CD5 nor CD10. At relapse, it was still CD5(-), but CD10 was now detected. Studies for a t(11;14) translocation and CYCLIN D1 protein expression, however, permitted a revised diagnosis of MCL. An MCL with this immunophenotype and classical morphology has not been previously reported.