By examining the endobronchial brushings from 65 infants and children with interstitial pneumonia--characterized by a severe hypoxia and diffuse lung infiltrates--we have detected the cysts of pneumocystis carinii in 17 of 21 subsequently confirmed cases. The rapid diagnosis and subsequent specific treatment have allowed the recovery of 18 of these children. The particular significance of this article is the youth of the patients, whose ages ranged from 4 months to 15 years.