Minor and reversible gastrointestinal side effects are common when patients receive interleukin-2 (IL-2) with or without lymphokine-activated killer (LAK) cells. We treated 42 cancer patients with IL-2 therapy and 3 patients developed serious gastrointestinal problems during treatment. Complications included sigmoid colon perforation, ischemic necrosis of the small and large intestine, and diffuse bowel ulceration. These were not associated with tumor implants or hypotension. Two patients died as a direct result of these problems despite aggressive surgical and medical management. The incidence of major gastrointestinal complications with IL-2 therapy may be greater than previously reported and a heightened awareness of potential gastrointestinal problems may circumvent considerable morbidity and mortality.