A study of the prevalence and incidence density of coal workers' pneumoconiosis (CWP) in West German coal miners was conducted. The cohort consisted of miners who had worked in coal mines in the Federal Republic of Germany and who had left mining in 1980 (548 miners) or in 1985 (404 miners). A total of 289 miners had worked underground for at least 5 years. Serial chest X-rays obtained during periodic medical examinations were read for evidence of CWP. Prevalences and incidence densities of all categories of CWP were computed for each 2 year interval of underground work from the data. The prevalence of simple CWP manifested by radiographic opacities of ILO category 1/1 or higher for miners who worked 28 to 30 years underground was 15.5%. The median ILO classification for all subjects who worked for 32 years was 0/0. The upper confidence limit of the median reached 0/1 in the last 2 year interval, 30 to 32 years. The 95th centile for simple CWP for the first 14 years of underground work was 0/0. After 14 years of underground work the 95th centile changed profusion category every 3.5 years on the average. The incidence density of simple CWP tended to increase with increasing time spent underground. After approximately 30 years of underground work the incidence density of simple CWP manifested by ILO categories 0/1, 1/0, 1/1, and 2/2 was approximately 7500, 4000, 2000, and 500 cases per 100,000 years exposure, respectively. The risk of developing 0/1, 1/0, 1/1, or 2/2 category simple CWP increased with increasing time underground. Only one case of progressive massive fibrosis was detected in the cohort. The authors conclude that the risk of developing simple CWP increases with increasing time spent in underground mining.