In France, both active and retired coal miners take part in medical surveillance programs. Those compensated for pneumoconiosis are registered and receive annual chest X-rays and regular lung function assessments. A longitudinal study was done among 2719 pneumoconiotics from the Nord-Pas de Calais region Compensation Register, who received first compensation between 1942 and 1987 to study progression of CSWP. Chest radiographs taken at time of compensation and in 1987 were examined by three independent readers. There was a change over time in the characteristics of pneumoconiosis at the time of first compensation toward a low profusion of irregular opacities. In the period from 1982 to 1987, 645 pneumoconiotics developed progressive massive fibrosis (PMF). The occurrence of PMF was related to the date of compensation and the profusion of small opacities at detection (after controlling for time to follow-up). Two profiles for changes in coal workers' simple pneumoconiosis (CWSP) were observed: the first in the group of subjects with mild pneumoconiosis at compensation, who did not reach category 2 at follow-up and had a low attack rate of PMF; and the second in the group of those compensated for category 1/2 pneumoconiosis or higher, who reached severe CWSP and had a twofold attack rate for PMF at follow-up. The changes observed in the characteristics of pneumoconiosis at first compensation between 1942 and 1987 suggest a lessening of disease severity.