Study of a patient with the bullous delayed pressure urticaria syndrome showed remarkable congruence of the extra-cutaneous findings and the known effects of interleukin 1: malaise, fever, myalgia, arthralgia, leukocytosis, increased sedimentation rate, and circulating acute phase reactants. As a result of this "clinical assay" for interleukin 1, we conclude that the delayed pressure urticaria syndrome is the clinical expression of interleukin 1, synthesized in the skin as a result of pressure and released into the circulation. Delayed urticaria, mediated by interleukin 1, is to be contrasted with immediate type urticaria, long known to be histamine mediated.