Preliminary results of the fourth round of the National Study of Coalworkers' Pneumoconiosis were discussed. This was the fourth in a series of medical surveys of workers at 31 coal mines distributed across the United States that started in August 1969. The cohort for round-4 consisted of 7396 miners that had participated in round- 1 and round-2 who were no older than 45 or 48, respectively, at the time. Of the cohort members, 4712 were selected for examination. Approximately 68% of these participated, excluding those who had died, moved, or who had valid reasons for not participating. The subjects who refused to participate had the longest underground exposure, the greatest prevalences of chronic airway obstruction and coal workers' pneumoconiosis, higher frequencies of respiratory symptoms, and smoked more than the participants. The subjects who did not participate because they had moved had large numbers of smokers, but tended to have the fewest signs of lung disease. Analyses of possible biases, including an analysis based on awards of black lung disability benefits, indicated that although bias may be present due to nonparticipation of less healthy subjects the degree of bias should not be large. The author notes that approximately 60% of the selected round 4 cohort has been examined. When compared with earlier data the cohort appears to be in better health than those who did not participate.