Forty-five children with anaphylactoid (Henoch-Schönlein's) purpura were studied. Evidence of recent streptococcal infection, as determined by positive ASO titers, was found in 13 of 39 patients (33%), an incidence of positive ASO titers only slightly higher than that expected in children with nonstreptococcal illness. This observation suggests that streptococcal infection is not the major cause of anaphylactoid purpura. Microscopic study of skin biopsy specimens from 12 children showed perivascular cellular infiltration of the dermis. Numerous leukocyte-platelet thrombi were observed in small dermal vessels. Muscle biopsy specimens were usually normal. Kidney biopsy specimens from 11 children with anaphylactoid purpura-nephritis of variable severity and duration were studied. The principal microscopic abnormality consisted of focal lesions of fibrinoid deposition and endothelial proliferation within scattered glomeruli. Renal biopsy specimens obtained during clinical recovery from anaphylactoid purpura-nephritis showed focal glomerular scars. The differences between the glomerular pathology of anaphylactoid purpura-nephritis and other forms of glomerulonephritis are discussed. Clinical, experimental and pathologic data are reviewed which support the concept that anaphylactoid purpura is a form of diffuse vascular disease, probably caused by hypersensitivity to a variety of agents.