Cytokines are important regulators of acute myelogenous leukemia (AML) blast proliferation. For a subset of patients the AML blasts show constitutive cytokine secretion and can undergo autonomous proliferation in vitro, whereas for other patients the blasts are dependent on exogenous cytokines for proliferation. The capability of autocrine proliferation is an adverse prognostic factor in AML. The three cytokines interleukin (IL)-4, IL-10 and IL-13 modulate in vitro blast proliferation, but the final effect of each cytokine (enhancement/inhibition/no effect) depends on differences between individual patients as well as the presence of other exogenous cytokines. In contrast to these divergent effects on blast proliferation, all three cytokines inhibit constitutive in vitro cytokine secretion by AML blasts. Because of the divergent effects on blast proliferation, it seems less likely that clinical therapy with these cytokines can be used to directly modulate AML blast proliferation. However, their effects on normal immunocompetent cells (and possibly the antigen-presenting capacity of AML blasts) are easier to predict. Thus direct (therapy with exogenous IL-4, IL-10 or IL-13) or indirect (enhancement of endogenous release of IL-4, IL-10 or IL-13) modulation of these cytokine effects on immunocompetent cells may become a useful clinical approach for enhancement of antileukemic immune effects. Such a modulation of immune reactivity can be used either as in vivo patient therapy or as manipulation of stem cell grafts prior to transplantation.