Portuguese man-of-war stings occur frequently at North American beaches. Most produce only local pain and pruritis. A 4-year-old girl who developed a hemolytic reaction and acute renal failure following a severe sting is described. As neither sequela has been previously reported, it is speculated that either the large dose of venom or chance intravenous inoculation may have been responsible. Evidence of hemolysis should be sought in small children with extensive Portuguese man-of-war stings.