The majority (85%) of patients receiving high dose Interleukin-2 (IL-2) experience gastrointestinal side effects, namely nausea, vomiting and diarrhea. In this article we describe four cancer patients that developed localized necrosis or perforation of the colon during IL-2 treatment. They represent 1.3% of patients (n = 315) or 0.9% of treatment courses (n = 452). All underwent surgical intervention and survived to leave the hospital. No common etiologic factor was apparent. Two patients were subsequently retreated with IL-2 without complication. Localized colonic necrosis or perforation is a rare complication associated with high dose IL-2 treatments. Aggressive surgical intervention is advocated and retreatment following recovery is safe.