Cytokines are potent modulators of the physiological immune response. A number of reactive processes are associated with a deregulation of this cytokine expression. Animal models with transgenic mice or transfection experiments have shown that an autocrine or paracrine pathway might be involved in tumor progression and/or tumorigenesis. The study of cytokine expression in malignant lymphomas showed a heterogeneous expression pattern with a number of cases which quantitatively show indications for a marked deregulation of the cytokine expression. The expression od IL-7 and IL-9 seem to be special features of Hodgkin's disease and large cell anaplastic lymphomas. Transfection experiment with IL-9 into mouse T cells results in an autocrine loop and tumorigenesis of the transfected cells. These tumors share a number of similarities to Hodgkin's disease and large cell anaplastic lymphomas in human.