Because of an increasing number of patients suffering from Leishmania infections and because of the serious consequences of these infections more thorough knowledge of the host factors responsible for resistance and susceptibility to the diseases is needed. In murine models of Leishmania infections the cytokine production by CD4+ T cells has been identified as a major factor in determining the outcome of the infection. In these models Th1 cells producing IFN-gamma provide protection against the infection whereas Th2 cells producing IL-4 and IL-10 aggravate the disease. The fatal outcome of Leishmania infections in humans with defects in T-cell functions illustrates that these cells are fundamental in the defence against Leishmania in humans also. However, as for many other infectious diseases (meningococcal disease and other septicaemic conditions, pneumonia, viral hepatitis, schistosomiasis) the immune reactions to Leishmania parasites in humans can be associated with both protection and pathogenesis. Many individuals without previous exposure to Leishmania parasites have T cells which can respond to Leishmania antigens. These cells have the potential to generate either Th1 or Th2 like responses. During infection with Leishmania parasites humans develop specific T-cell recognition of well-characterized parasite antigens. T cells producing disease-exacerbating factors such as IL-4 in response to Leishmania antigen stimulation have been identified in humans as well as in mice. Both Th1 like and Th2 like cells recognizing Leishmania antigens can be expanded during infection. At the polyclonal level Th1 like responses to Leishmania antigens are found in individuals who have had self-healing or asymptomatic infections. Factors secreted by such Leishmania specific Th1 like cells can induce killing of intracellular parasites in infected macrophages. In individuals who have been cured from uncontrollable disseminating disease both Th1 and Th2 like responses can be detected. A restriction in the antigen recognition to particular protein fractions could not be demonstrated in the Th1 or Th2 like responses. These findings suggest an association between the pattern of cytokines produced by parasite specific T cells and the clinical course of the infection similar to the one seen in mice. In the murine model the cytokine pattern present in the animal at the time of infection can determine whether a Th1- or a Th2 response will develop. In vitro studies on human and murine cells have confirmed that certain cytokines (e.g. IFN-gamma, IL-12) will favour maturation of Th1 responses whereas others (e.g. IL-4, IL-10) support Th2 development. If similar immunoregulatory mechanisms operate in mouse and man, design of vaccines against human leishmaniasis should aim at introducing powerful Th1 like responses. Importantly, once generation of either Th1 or Th2 has started, the immune response seems to be locked in this pattern, even when it is harmful to the host. Therefore new vaccines against leishmaniasis should be designed in a way that they generate controlled Th1 like primary responses.