The authors performed immunohistochemical and cytogenetic studies in a 73-year old man with malignant angioendotheliomatosis. The patient was referred for evaluation of fever of unknown origin, hepatic failure, and neurologic deterioration. Examination of a muscle biopsy revealed numerous, noncohesive atypical mononuclear cells within small vessels. These cells stained positively with a pan-leukocyte marker CD45(PD7/26/16) and with a B-cell marker L26 but negatively with Factor VIII-related antigen, an endothelial cell marker. Peripheral blood obtained before chemotherapy was cultured and analyzed by the G-band method. A new translocation and numerous chromosomal aberrations were identified. The major cell line karyotype was 53,XY, +X, +5q?,-6, +i(6p), +7, -10, +11, -12, +12p-, +12p-, +18, +mar1, +mar2, t(1;3)(p22;p21),3q+,8p+. This is the first cytogenetic study performed in a case of malignant angioendotheliomatosis. Our findings demonstrate that the neoplastic cells in this disorder circulate in the peripheral blood and provide further evidence that malignant angioendotheliomatosis is a diffuse intravascular neoplasm of lymphoid origin. Furthermore, the authors conclude that this malignant lymphoproliferative disorder should be reclassified as a primary intravascular (angiotropic) lymphoma.