The mechanical properties of the lungs were studied in two groups of coal miners. The first group consisted of miners with either simple or no pneumoconiosis and was divided into two subgroups (1A and 1B). The former (1A) consisted of 62 miners most of whom had simple pneumoconiosis but a few of whom had clear films. Although their spirometry was normal, all claimed to have respiratory symptoms. The other subgroup (1B) consisted of 25 working miners with definite radiographic evidence of simple pneumoconiosis but normal spirometric findings. The second major group consisted of 25 men with complicated pneumoconiosis. In subjects with simple pneumoconiosis, static compliance was mostly in the normal range, whereas it was often reduced in subjects with the complicated disease. The coefficient of retraction was normal or reduced in most subjects except those with advanced complicated disease, in several of whom it was elevated. So far as simple pneumoconiosis was concerned, abnormalities, when present, reflected "emphysema" rather than fibrosis. In severe complicated pneumoconiosis, changes suggesting fibrosis tended to predominate. In the 25 working miners (subgroup 1B) dynamic compliance was measured at different respiratory rates. 17 of the subjects in this subgroup demonstrated frequency dependence of their compliance, a finding unrelated to bronchitis and suggestive of increased resistance to flow in the smallest airways.