Insoluble materials isolated by bronchoalveolar lavage from the lungs of four patients with alveolar proteinosis were examined under the electron microscope, categorized according to ultrastructural criteria, and quantitated using morphometric procedures. The major component of the insoluble accumulations was the tubular myelinlike multilamellated structure that accounted for 42.6 +/- 12.4% of the total volume of insoluble material. These structures resembled tubular myelin as found in normal human lungs, consisting of lipid bilayer membranes separated by amorphous proteinaceous material; however, tubular myelinlike multilamellated structures lacked the intersecting membranes of normal tubular myelin. The distance between membranes varied from 150 to 300 A and this was similar to that found in tubular myelin from normal human lungs. Fused-membrane structures that were multilamellated but were without intervening amorphous lamellae accounted for 2.7 +/- 0.6% of the total volume of insoluble material. Crystals, identified by the presence of adhering materials, were present only in small amounts, accounting for only 0.07 +/- 0.03% of the volume of insoluble material. Neither fused-membrane structures nor crystals are present in lavage effluents from the lungs of normal humans. Membranous vesicles and electron-dense bodies, which resembled cell debris, together accounted for 21.2 +/- 10.1% of the total volume of insoluble material. Constituents of the pulmonary extracellular lining of normal human lungs such as secreted lamellar bodies and tubular myelin were minor components, together accounting for only 2.2 +/- 3.2% of the total volume of insoluble material. Amorphous material and miscellaneous structures accounted for 31.4 +/- 5.0% of the volume of insoluble material. Cellular disintegration and extracellular formation of unusual tubular myelinlike multilamellated structures appear to be major processes leading to the accumulation of insoluble materials in the alveoli and distal airways of patients with pulmonary alveolar proteinosis.