The sputa of 26 of 100 patients with chronic obstructive pulmonary disease contained microliths. The numbers of microliths per smear of sputum per patient ranged from one to 20. In seven of the sputum specimens containing microliths, deposition of calcium was present on the loops of Curshmann's spirals to different extents, from a single layer to the laminated appearance seen in a free microlith. Partial and complete separation of the calcified laminated masses from the spirals were also demonstrated. Based on these observations, the morphogenesis of microliths and the pathogenesis of pulmonary alveolar microlithiasis are discussed.