While the number of reported cases of thrombotic thrombocytopenic purpura, first described by Moschcowitz in 1925, has steadily increased during the past 15 years, the diagnosis has been established in most instances only on postmortem demonstration of the characteristic vascular lesion. Although clinical suspicions are now frequently aroused by patients presenting with evidences of progressive disseminated disease including thrombocytopenia, hemolytic anemia, transitory disturbances of the central nervous system, fever, and mild to moderate nephropathy, definite studies of the pathogenesis of this unexplained disorder have been hampered by the infrequency with which a conclusive diagnosis is established. Bernheim,1 in 1943, suggested that since necropsy had revealed the presence of widespread vascular lesions in a case of thrombotic thrombocytopenic purpura, antemortem biopsy might have established the diagnosis. The site for such biopsy was not indicated. Fitzgerald and co-workers,2 in 1947, reported an additional 3 cases, in each of which necropsy