In this study we compared the effects of IL-2, IL-15, and IL-21 on the gene expression, activation of cell signaling pathways, and functional properties of cells derived from the CD4+ cutaneous T-cell lymphoma (CTCL). Whereas both IL-2 and IL-15 that signal through receptors that share the common gamma chain and the beta chain modulated the expression of >1,000 genes, IL-21 that signals via the receptor also containing gamma chain up-regulated <40 genes. All three cytokines induced tyrosine phosphorylation of Jak1 and Jak3. However, only IL-2 and IL-15 strongly activated STAT5, PI3K/Akt, and MEK/ERK signaling pathways. In contrast, IL-21 selectively activated STAT3. Whereas all three cytokines protected CTCL cells from apoptosis, only IL-2 and IL-15 promoted their proliferation. The effects of the cytokine stimulation were Jak3- and Jak1-kinase dependent. These findings document the vastly different impact of IL-2 and IL-15 vs. IL-21 on malignant CD4+ T cells. They also suggest two novel therapeutic approaches to CTCL and, possibly, other CD4+ T cell lymphomas: inhibition of the Jak1/Jak3 kinase complex and, given the known strong immunostimulatory properties of IL-21 on CD8+ T, NK, and B cells, application of this cytokine to boost an immune response against malignant CD4+ T cells.