This study comprises 31 cases of infantile Pneumocystis carinii (P.C.) pneumonia diagnosed by autopsy or complement fixation test. The authors distinguish 2 types of infantile P.C. infection: a focal type with benign course; and a diffuse fulminating type, which is usually fatal.It is concluded that the infantile P.C. may be suspected, but can not be diagnosed upon a single roentgenographic examination with or without clinical and/or laboratory support. The diagnosis can, however, be closely approximated by evaluation of the roentgenologic changes and appraisal of the clinical course of the disease.The roentgenographic changes in focal P.C. infection are not characteristic and are basically determined by associated pulmonary infiltrations due to concomitant interstitial processes, mostly of viral etiology.