Multiple sequential biopsy specimens of wheals elicited by a solar simulator in four patients with severe solar urticaria were studied with the use of indirect immunofluorescence for eosinophil granule major basic protein. Examination of control biopsy specimens from normal, unstimulated skin did not show eosinophils or major basic protein deposition. Five minutes after solar simulation, eosinophils were observed in vessels in the dermis, and 2 hours later there was marked tissue eosinophilia. Extracellular major basic protein was extensively deposited in the dermis at 2 and 24 hours, suggesting eosinophil degranulation. Thus evolution of the solar urticaria wheal is accompanied by infiltration of eosinophils and neutrophils and by tissue deposition of the eosinophil major basic protein, suggesting eosinophil degranulation.