Irukandji syndrome is a constellation of delayed severe local and systemic symptoms occurring after a Carukia barnesi box jellyfish sting involving any exposed skin. These cases are limited to Australia, the habitat of that animal. Numerous other cases of an Irukandji-like syndrome after other small Carybdeid genus envenomations have been reported elsewhere in the world. There have yet been no reports of Irukandji-like syndrome occurring in continental US coastal waters. We describe 3 cases of marine envenomation causing such a symptom complex in US military combat divers off Key West, FL. It is unclear what species caused the injuries, but a member of the Carybdeid genus seems most likely.