A native-born American infant presented clinical and pathologic features characteristic of pneumonia due to Pneumocystis carinii. Apparently at the present writing this is the second such case to be reported from the United States. Though the clinical picture was distinctive enough to suspect the etiology during life, confirmation awaited pathologic studies. This is the third known case to show a low concentration of gamma globulin as a possible contributory factor. Much remains to be learned about this distressing but apparently quite distinctive pulmonary disease, which may be more common than is now generally appreciated.