Alveolar microlithiasis is a rare idiopathic alveolar filling disorder. It is characterized by intra-alveolar accumulation of calcium phosphate, which forms laminated concretions. Microscopically there are heavily calcified laminar microspheres within the alveolar spaces. These lesions progress slowly and may reach diameters of 0.01 to 3.0 mm. Fibrosis develops over time and eventually the lungs become rock hard. It has been speculated that the disease may be due to a genetically determined metabolic disorder or acquired defects in calcium and phosphorous metabolism. A familial association has been described, but some cases are sporadic. Some authors have proposed an autosomal recessive pattern of inheritance. Alveolar microlithiasis is seen in the third to fifth decades, although it can occur in children. There is no sex predilection. Occasionally microliths can be found by bronchoalveolar lavage. Grossly, the lungs appear gritty and hard. Primary differential diagnosis includes corpora amylacea and intra-alveolar lamellar bodies commonly seen in the lungs of older individuals or patients with left heart failure. Corpora amylacea are not calcified or ossified, although they are birefringent with polarized light, and they consist of carbohydrates.