The National Institute for Occupational Safety and Health administers the X-ray Screening Program for underground coal miners, a program mandated by the Federal Mine Health and Safety Act of 1969. The screening file, with over 200,000 x-ray films, affords an excellent source for the study of prevalence and progression of Coal Workers' Pneumoconiosis. Two epidemiological analyses have recently been completed. One, a prevalence study of miners with 10 or fewer years of mining tenure, converted screening readings to median epidemiological readings. Converted prevalences were 0.44% for the group with 0 to 1 year tenure and 0.79% for the group with 1 to 9 years tenure. This result is similar to prevalence observed in a study of nonexposed blue collar workers. A second analysis reread x-ray films of a subgroup of 1,834 repeat miners with roughly 9 years exposure only under mandated dust standards. Net progression from category 0/0 was observed to be 1.2%. This value is consistent with 1.9%, based on an average dust exposure, predicted by British research. Results must be interpreted in light of several possible sources of bias.