Thin-section computed tomography (CT) was performed in 244 patients with infiltrative lung diseases and 29 healthy control subjects to evaluate the frequency, profusion, and diagnostic value of subpleural parenchymal micronodules. These areas of increased attenuation (less than 7 mm in diameter) were analyzed in four groups: coal miners with chest radiographic findings of coal worker's pneumoconiosis (n = 61), coal miners with no radiographic evidence of pneumoconiosis (n = 73), patients with nonoccupational chronic infiltrative lung disease (n = 110), and healthy adults (n = 29). Subpleural parenchymal micronodules were observed with high frequency in pulmonary lymphangitic carcinomatosis, coal worker's pneumoconiosis, and sarcoidosis but were also seen in 14% of control subjects. Predominant sites of lesions were the posterior subpleural areas in the upper lobes. Subpleural parenchymal micronodules have no diagnostic value when observed as an isolated CT finding but may suggest that diagnosis of pneumoconiosis, sarcoidosis, or pulmonary lymphangitic carcinomatosis when observed in association with mild parenchymal lesions.