Papular urticaria is the result of hypersensitivity (idreaction) to bites from certain insects such as mosquitoes gnats, fleas, mites, and bedbugs. Papular urticaria is common in childhood and is characterized by symmetrically distributed pruritic papules and papulovesicles. Scratching causes erosions and ulcerations. Pyoderma is common. Lesions occur in crops. The histopathologic features of papular urticaria are inadequately documented. In a prospective study we recorded the histopathologic features of 30 patients (female, 18; male, 12) with papular urticaria. Their ages ranged from 6-343 months (median = 21 months, mean = 37.73 months). Features that presented in more than 50% of cases included mild acanthosis, mild spongiosis, exocytosis of lymphocytes, mild subepidermal edema, extravasation of erythrocytes, a superficial and deep mixed inflammatory cell infiltrate of moderate density, and interstitial eosinophils. We recognized lymphocytic (n = 4), eosinophilic (n = 9), neutrophilic (n = 7), and mixed (n = 9) subtypes. Immunohistochemistry was performed on formalin-fixed, paraffin-embedded sections from 10 cases and revealed abundant T-lymphocytes (CD45RO, CD3) and macrophages (CD68) in all cases. B-lymphocytes (CD20) and dendritic antigen-presenting cells (S100) were absent. Direct immunofluorescence staining was conducted on cryostat-prepared sections from 26 specimens. Deposition of IgA, IgG, IgM, C3, and fibrin could not be demonstrated. The histopathologic differential diagnosis of papular urticaria includes other spongiotic dermatitides, pityriasis lichenoides et varioliformis acuta, the pruritic papular eruption of human immunodeficiency virus disease, and papulonecrotic tuberculid. Papular urticaria with marked spongiosis and a dense inflammatory cell infiltrate cannot be reliably distinguished from arthropod bites on clinical and histopathologic grounds. The present study provides morphologic and immunohistochemical evidence that a type I hypersensitivity reaction plays a central role in the pathogenesis of papular urticaria. The putative antigen remains undetermined.