The wheal-and-flare response to intradermal autologous serum in chronic urticaria offers a model for study of the pathogenesis of the disorder. Serial biopsies of autologous serum induced wheals were performed in 5 chronic urticaria patients to assess the evolution of the cellular inflammatory response and to look for evidence of mast cell degranulation. Perivascular neutrophils and eosinophils were seen as early as 30 min, becoming more intense and diffuse over 2 h. T lymphocyte numbers were increased by 2 h, CD4+ cells outnumbering CD8+ cells at 24 h. By 48 h, the neutrophils were clearing, but eosinophils and lymphocytes persisted. The histology of compound 48/80-induced wheals was similar to serum-induced wheals, but there was little or no response to physiological saline (0.16 M). Stainable mast cells were reduced in compound 48/80- and serum-induced wheals when compared to saline skin tests. Mast cell granules appeared swollen and had lost their characteristic lamellar substructure on electron microscopy of a serum-induced wheal biopsied at 10 min. Eosinophil degranulation was also observed at 2 h. The resemblance of the inflammation to the late phase of IgE-mediated immediate hypersensitivity reactions in atopics supports the concept that a circulating factor causes mast cell degranulation in chronic urticaria and may be important in the pathogenesis of the disorder.