CD137 (4-1BB) is a surface marker discovered on activated T lymphocytes. However, its expression pattern is broader and has also been described on activated NK cells, B-cells and myeloid cells including mature dendritic cells. In this study, we have immunostained for CD137 on paraffin-embedded lymphoid tissues including tonsils, lymph nodes, ectopic tertiary lymphoid tissue in Hashimoto thyroiditis and cancer. Surprisingly, immunostaining mainly decorates intrafollicular lymphocytes in the tissues analyzed, with only scattered staining in interfollicular areas. Moreover, pathologic lymphoid follicles in follicular lymphoma and tertiary lymphoid tissue associated to non-small cell lung cancer showed a similar pattern of immunostaining. Multicolor flow cytometry demonstrated that CD137 expression was restricted to CD4+ CXCR5+ follicular T helper lymphocytes in tonsils and lymph nodes. Short term culture of lymph node cell suspensions in the presence of an agonist anti-CD137 mAb or CD137-ligand results in the functional upregulation of TFH cells, including CD40L surface expression and cytokine production, in three out of six cases. As a consequence, immunostimulatory monoclonal antibodies, anti-CD137 mAb such as urelumab and PF-05082566 should be expected to primarily act on this lymphocyte subset, thus modifying ongoing humoral immune responses.