In two mines, "A"--in Lower Silesia with low, inclined coal seam and lesser degree of output mechanization and "B"--in the Silesia-Dabrowa Basin with high coal seams and a great degree of output mechanization), work conditions were examined (dust concentration, chemical composition of dust, and microclimate) and medical examinations of underground miners were done. The examinations involved: mine "A"--2094 miners, mine "B"--1846 miners. The two examined populations were almost identical as regards age and length of employment. The number of miners working in bad conditions in the "A" mine was twice as great as that of miners working in bad conditions in the "B" mine. In dust collected on work--stands in the "A" mine the amount of copper, chromium and vanadium was 3 times that in the "B" mine and in the "A" mine approximately 40% more nickel was found in the dust than in the "B" mine. Average dust concentration on work stands was 10 times greater in the "B" mine than in the "A" mine. The rate of pneumoconiosis was on average 3.7% in both populations, in the "A" mine--6.2% and in the "B" mine--0.8%. In currently working miners no nodular changes, like PMF, were found. The rate of pneumoconiosis significantly varied with the length of employment. In 71.7% miners changes in EKG were found, in the "B" mine the number of those with changes being about 10% greater than in those in the "A" mine. In miners from the "A" mine, mostly changes in conduction (17.1%) were found; in the "B" mine these changes were twice less frequent.