Study of acute infant lung infections through transthoracic pulmonary aspiration. In the period of April 1979 to July 1980 were studied 45 children of both sexes aged from 2 months to 5 years, selected from patients at the "INSTITUTO DA CRIANÇA DO HOSPITAL DAS CLÍNICAS" of the, "FACULDADE DE MEDICINA DA UNIVERSIDADE DE SÃO PAULO", and suffering from a acute lung infection. Of the 45 patients studied, the majority (46.7%) were in the age group of 2 to 12 months and, in terms of nutritional conditions, 42.2% were in eutrophic condition and 57.8% were undernourished, with this malnutrition varying from first to third degree. By using samples obtained through pulmonary aspiration and examining them through Gram staining and culture, it was possible to identify the etiological agent in 26 (57.8%) bacillary pneumonias of the 45 cases studied. Direct identification by the Gram staining method proved useful for initial orientation of antibiotic therapy in 44 4% of the cases. There was a distinct predominance of Streptococcus pneumoniae as the bacillary etiological agent in the acute infant lung infections of the various age groups studied, followed by Haemophilus influenzae and Staphylococcus aureus. The comparative study of cultures of pulmonary aspirate and hemocultures permitted greater diagnostic precision (56.7%), demonstrating a positive hemoculture in only 30.0% of the cases. There was no evidence of participation by anaerobic germs in the lung infections of the samples estudied.