A histopathologic study four cases of Pseudomonas aeruginosa septicemia in the newborn is presented. All of these cases refers to prematures born in fairly good conditions, their symptomatology having appeared after the third day of life, a fact which accounts for the extra-uterine origin of the disease. In all of them peculiar visceral lesions were observed extensive areas of necrobiosis crowded by microorganisms, along with hemorrhagic ones, free from inflammatory reaction and always accompanied by a necrotic vascularitis. The accumulation of myriads of delicate bacilli on the media and adventitia of the blood vessels confers to the attained vessels a peculiar aspect, which is commonly considered as pathognomonic of the Pseudomonas septicemia. Probably the digestive lesions should occur in the first place, as they proved to be largely disseminated and extensive, emphaszing the involvement of the tongue in two cases. The esophasus was severely attained in three cases and the stomach only in one. Intestinal lesions were constantly observed, showing a peculiar macroscopic aspect, seeming to be typical of Pseudomonas infection. The gross pulmonary lesions had no specific aspect, and only in one case a diagnostic could be assumed when simultaneous digestive lesions were also taken into consideration. Furthermore microscopic examination revealed typical lesions in two cases, absence of specific lesions in another case and presence of the interstitial type of pulmonary Pseudomonas infection in the last one. Splenomegaly was always oberved; typical microscopical lesions having ben verified in two cases. Macroscopic examination of the liver was inexpressive and microscopic inspection showed typical necrobiotic areas only in one case, accompanied by specific vascularitis. It is noteworthy in all the cases the absence or scarce hepatic erythopoiesis, a fact we attribute to the depletion of the bone marrow. Hepatic bile stasis and hemossiderosis were constantly verified. It was remarkable that the presence of kernicterus occured in three cases, the cause of which may be linked to the hemolytic and neurotoxic action of the Bacillus pyocyaneus toxins.