The incidence and progression of pneumoconiosis over 9 years in US coal miners was examined and compared to British findings. The study, which began in 1970, consisted of over 9000 miners followed up for 9 years the final cohort including 1470 subjects. The loss of miners was due mainly to retirement and mine closures. Examinations involved posterior/anterior and lateral X-rays. X-ray readings were obtained using the side by side method, thus giving readers an opportunity to compensate for technique differences, and ensuring comparability with the dust standard based on British data. Only small rounded opacities and large opacities were of concern in this report, and a summary reading was obtained for each abnormality. Progression rates from 0/0, 0/1, and 1/0 were 1.9, 13.6, and 18.2 percent, respectively. Incidence and progression of small, rounded opacities were consistent with values predicted from dose response curves developed by the British interpolation at a concentration derived from US compliance standards. Results showed that the incidence of pneumoconiosis in US miners was reduced through application of the current 2 milligrams per cubic meter (mg/m3) standard. The authors conclude that because of the long developmental period for pneumoconiosis, observation of the consequences of a lifetime exposure to 2mg/m3 will be necessary for the validity of the standard to be verified as an appropriate regulatory dust exposure concentration.