BACKGROUND. We tested the hypothesis that patients who have recovered from leptospirosis have peripheral blood memory T cells that are specific for Leptospira or Leptospira protein antigens. METHODS. Peripheral blood mononuclear cells (PBMCs) were obtained from patients who had recovered from leptospirosis, as well as from control individuals. PBMCs were assessed for in vitro proliferation, phenotyping, and cytokine production after stimulation with different strains of Leptospira, recombinant LipL32, or overlapping synthetic peptides of different outer membrane proteins. RESULTS. PBMCs from both control subjects and patients produced significant proliferative responses to all Leptospira strains. Proliferation from control PBMCs was significantly greater than responses produced by patient PBMCs. Select strains of Leptospira expanded both T cell receptor (TCR) alphabeta and TCRgammadelta T cells in both control subject and patient PBMCs. Patient and control subject PBMCs produced equivalent levels of tumor necrosis factor alpha and interferon gamma, but patient PBMCs produced significantly less interleukin 10 than did control subject PBMCs after stimulation by different strains of Leptospira. PBMCs from patients failed to respond to recombinant LipL32 or to any of the Leptospira peptides. CONCLUSION. Leptospira induced significant proliferative responses, TCRgammadelta T cell expansion, and cytokine production in both control subject and patient PBMCs. Patient PBMCs failed to recognize Leptospira protein antigens. Leptospirosis does not seem to generate memory T cells that can be activated by in vitro stimulation.