An unusual type of infantile pneumonia occurs frequently in certain European countries, but thus far is undescribed in the American literature. It is remarkable clinically in being almost asymptomatic until late in its course, in showing a marked predilection for premature or immature infants, and for being confined to the age period of 6 weeks to 4 months. Pathologically, it is remarkable for a diffuse interstitial pulmonary infiltration by mononuclear cells which, for the most part, closely resemble plasma cells. An alveolar exudate is also present. About 22% of patients with clinically recognized cases die of asphyxia. In these cases death appears to be predominantly due to asphyxia. The most commonly held opinion, in which the authors concur, is that the disease is infectious and probably of viral origin, but as yet there has not been a convincing demonstration of the etiologic agent. Up to the present time no effective therapeutic agent has been found.