SummaryFour cases of«septicemia tuberculosa acutissima»are reported. Characteristic necrotic abcesses (without the common signs of focal reaction : Lang- ans’s and epetheloid cells) were found in the lungs, the liver, the spleen, the kidneys, the lymph nodes, the bone marrow, and even the nervous system.Hematologic changes were observed in all cases. In one, this type of tuberculous disease appeared as a complication of a chronic myeloid leukemia. In two other cases the infection developped together with signs of acute leucemia and it was impossible to recognize which of these disease appeared to be the primary one. In the fourth patient evidence was obtained that tuberculosis acutissima was the primary disease, acute leucemia appearing as a terminal complication responsible for death.Pathological examination disclosed extensive tuberculosis of the lymph nodes in three cases. Large fibrous scars were found in visceral nodes in three cases and, besides, in peripheral nodes in two of them.As a hypothesis, it is suggested that the extensive lymphatic involvment was responsible for the necrotic form of tuberculosis observed in these patients. Several experimental observations suggest that lymphoid tissue plays an important role in tuberculous defense : irradiation of lymphoid tissue as well as cortisone therapy increase the severity of experimental infection in animals. Besides, extensive tuberculosis of the lymph nodes is frequently associated with hematologic changes. Therefore it may be suggested that the large reduction of lymphoid tissue in our cases was responsible for the particular features of the tuberculous infection as well as for the leucemic changes of the bone marrow.