Interleukin-1 (IL-1) is a cytokine with many activities central to immune function and hematopoiesis. Many IL-1 properties can be potentially exploited in the treatment of malignancy. This review describes the toxicities and antitumor effects observed in Phase I and II trials of IL-1 in cancer patients. The immunophysiology and the induction of cytokines by IL-1 has been examined in many of the Phase I trials and has aided in understanding IL-1 effects in humans. The influence of IL-1 on granulopoiesis and thrombopoiesis when given by different regimes is also reviewed in detail.