The CD4(+) T cell-mediated inflammatory response to Pneumocystis carinii (PC) critically contributes to the clinical severity of PC pneumonia. It has been suggested that lymphopenic conditions predispose individuals to this immunopathology, although the mechanisms remain poorly understood. Another set of evidence indicates that a subpopulation of CD4(+) T cells constitutively expressing the CD25 molecule prevent lymphopenia-induced autoimmunity and inflammatory bowel disease. We tested the ability of this CD25(+)CD4(+) population to regulate CD4(+) T cell-mediated inflammatory response to PC. Adoptive transfer of CD25(-)CD4(+) cells into PC-infected recombination-activating gene-2-deficient mice led to lethal pneumonia within 13 days post-transfer. PC infection appeared to trigger CD25(-)CD4(+) cells, since recipients with reduced PC load survived up to 5 weeks after transfer. In contrast, transfer of CD25(+)CD4(+) cells did not induce lethal pneumonia and prevented the development of the disease induced by CD25(-)CD4(+) cells. Furthermore, CD25(-)CD4(+) cells reduced the PC load in the lung, while CD25(+)CD4(+) cells suppressed this immune response. Our results indicate an essential role for CD25(+)CD4(+) T cells in the control of PC-driven immunopathology, and suggest that in immunocompromised hosts PC pneumonia may result from a deficiency in regulatory T cells.