A protocol for etiologic diagnosis has been used on 40 immuno suppressed children with pulmonary infiltrate. The purpose of this protocol is to permit an early etiologic diagnosis, concomitant infections by several pathogen agents being frequently seen. An endobronchial brushing is first performed; the results being obtained between 24 to 48 hours. If the results are negative and the patient's condition worsens we propose an open lung biopsy. Thus pathogenic agent has been established in 24 cases, in 17 cases by endobronchial brushing, in 3 cases by open lung biopsy and in 2 cases by endobronchial brushing and open lung biopsy. In 3 cases several opportunistic organisms were responsible of the pneumopathy. Pneumocystis carinii is the most often established (19 cases); it was also present in one out of 6 post mortem examinations. In 14 cases the pneumopathy occured in children treated for acute leukemia; Pneumocystis was the organism responsible in 6 of them; the evolution was favorable in 13 of the 14 cases. One patient died from a pulmonary infection by the measles virus.