A case of localized urticaria in an otherwise healthy young woman, produced only by direct contact of the skin with heat, is described. The minimal temperature of urtication was 44°C (immersion of the forearm in hot water for 5 min). Redness and painful oedema immediately developed without reflex flare. Total serum IgG, IgA, IgM, IgE, complement factors C3 and C4, and α1-antitrypsin were in the normal range, whereas the C1-inhibitor level was slightly decreased. There was no evidence of circulating immune complexes in the serum. A skin test and a RAST with house dust were positive, but there were no signs of respiratory atopy. An attempt for passive transfer of heat urticaria into the abdominal skin of a rhesus monkey failed, but was successful for house dust. A treatment trial with ketotifen, a new, perorally acting anti-allergic drug, was poorly effective, but dexchlorpheniramine maleate, a classical antihistaminic, in a dose of 12 mg daily completely suppressed the swelling evoked by heat, but not the erythema, suggesting that other tissue or plasma factors than histamine may be involved in the mechanism of heat urticaria in this patien