A retrospective review of medical records from 113 patients with cnidarian stings in western O'ahu, Hawai'i, was conducted for the 5-year period 1994-98. The most common clinical feature was acute local pain, but cases of anaphylaxis or anaphylactoid syndrome and a persistent or delayed local cutaneous syndrome were also documented. Six cases resembled the Irukandji syndrome described from northern Australia, characterized by severe pain and signs of catecholamine excess, including muscle cramping, elevated blood pressure, diaphoresis, and tremor. Treatment with heat application, usually by means of a whole-body hot shower, appeared to provide better clinical improvement than parenteral analgesics or tranquillizers, particularly in patients with the Irukandji-like syndrome. The heat sensitivity of one or more of the Carybdea alata venom components might account for the effect of heat treatment. Prospective, randomized, controlled clinical trials should be performed to assess heat treatment for cnidarian envenomation.