Congenital alveolar proteinosis (CAP), a cause of respiratory failure in fill-term newborns, often leads to death in infancy despite medical therapy. We recently described an inherited deficiency of surfactant protein B (SP-B) (N. Engl. J. Med. 1993; 328:406-410) in two siblings with CAP. The SP-B deficiency was accompanied by marked abnormalities, both quantitative (increase) and qualitative (distribution), of SP-A and SP-C in the lungs of the affected infants. Ultrastructural studies of the lung of one of these infants and of a third affected sibling born in the index family showed abundant alveolar concentric multilamellated structures and membranous vesicles but no typical tubular myelin. In addition, membranous vesicles from type II cells and immunogold labeled SP-A and SP-C were found between type II cells and their basement membrane despite intact interepithelial cell junctions. These findings suggest an important role for SP-B in the directionality of surfactant secretion and in the formation of tubular myelin.