Most inbred strains of mice are susceptible to Leishmania amazonensis infection and develop progressive cutaneous lesions. However, the role of Th subsets in the disease and the molecular basis of pathogenesis are unclear. To address this issue, we examined the frequency of cytokine-producing CD4+ T cells and the profile of alphabeta T cell receptor (TCR) usage in infected BALB/c mice. At different infection stages, CD4+ cells of draining lymph nodes contained comparable frequencies of Th1 and Th2 cells, produced comparable levels of interleukin-4 (IL-4) and interferon-gamma in vitro, and showed no significant bias in aalphabetaTCR usage. However, T cells became highly polarized to a Th2 phenotype (IL-4+, IL-10+) within a few cycles of in vitro restimulation. These Th2 cells preferentially expressed Valpha2, Vbeta4, or Vbeta8.1/8.2, and significantly exacerbated disease in cell-transferred mice. Thus, unlike a Th2-dominant phenotype seen in L. major infection, a mixed Th1/Th2 response can be maintained in L. amazonensis-infected mice via an as-yet-unidentified mechanism.