The evolution of simple coal workers' pneumoconiosis (CWP) into progressive massive fibrosis (PMF) was examined. Selected posterior/anterior chest X-ray films from the NIOSH National Coal Study (NSCWP) and Coal Workers' X-ray Surveillance Program (CWXSP) were examined by an unblinded B-reader to find 5 year film pairs on individual miners that showed PMF on the later film but not on the earlier film. A total of 136 film pairs were identified, 115 from the CWXSP and 21 from the NSCWP. Only 69 films showed new abnormalities indicative of incident PMF. Of the 67 nonincident cases, 26 were thought to have PMF on both films. Eleven were considered to be borderline, but negative for PMF. Eighteen others were considered to show multiple or calcified solitary nodules. Among the films interpreted as indicating incident PMF, the prevalence of category 0, 1, 2, and 3 simple CWP in the earlier film was 14, 43, 33, and 9%, respectively. The prevalence of PMF on the later films was 1, 43, 38, and 17%, respectively. The prevalence of simple CWP p-type, q-type, and r-type rounded opacities on the PMF films was 9, 20, and 65%, respectively. The prevalence of s-type and t-type irregular opacities on the PMF films was 1 and 4%, respectively. The authors conclude that the high prevalence of category 0 and 1 simple CWP on the 69 films indicating incident PMF seems to corroborate British data, although none of the PMF cases have been confirmed pathologically. The rapid progression of the observed CWP lesions, particularly r-type opacities, suggests that free silica may be the important etiological agent in the study population and that strict dust control measures must be followed.