We report a case of localized heat urticaria in a 71-year-old woman who developed weals and loss of consciousness after taking a bath. Exposing her skin to heat at 40 degrees C or immersing her hands in water at 40 degrees C produced urticarial lesions and increased her plasma histamine level. Desensitization with hot water improved her symptoms and normalized her plasma histamine level after heat challenge. An intracutaneous injection of her serum produced no reaction, while an injection of her serum that had been heated at 40 degrees C for 15 min induced a weal flare response. Further examination revealed that the weal-inducing activity of her heated serum remained for at least for 6 h and that treatment of her serum at 60 degrees C for 2 h did not abrogate its weal-inducing activity. These findings indicate that certain materials in her serum that are activated by heat are responsible for the development of her anaphylactic and urticarial reactions and that these reactions may be mediated by histamine.