A study of time related trends in the prevalence of coal workers pneumoconiosis (CWP) in US coal workers was conducted. Data obtained by the Coal Workers X-ray Surveillance Program (CWXSP) were utilized. The CWXSP was a nationwide chest X-ray based medical monitoring program managed by NIOSH. The chest X-rays were read for CWP using International Contre le Cancer and 1971 and 1980 International Labour Office criteria. Data obtained during four rounds of the CWXSP, from 1970 to 1973, 1973 to 1978, 1978 to 1981, and 1981 to 1986, were reviewed to determine the prevalence rates of CWP as a function of the time spent working underground (tenure time). The number of participants in the program during the four rounds varied from 40,634 in the fourth round to 122,675 in the second round. During each round the prevalence of simple and more severe categories of CWP increased with increasing tenure time, the increases becoming significant after 5 to 9 years tenure. The prevalence of all categories of CWP decreased over time, the prevalence rate for all CWP grade during the fourth round being approximately half that during the first round. The authors conclude that the CWXSP data indicate a clear downward trend in the prevalence of CWP among the program participants. The decreases in CWP prevalence coincide with the lower coal mine dust exposures mandated by the 1969 Federal Coal Mine Health and Safety Act.