A study was made of the size distribution of dust particles in autopsied lungs of 60 coal miners. Digestion with hydrogen-peroxide and microincineration were used to obtain particles for microscopic analysis. Two random samples were taken from each of the autopsies. Dust particles were divided into mineral and coal categories and sized. For mineral dust particles, 55.6% were less than 2 microns in diameter, 25.8% were 2 to 5 microns, 10.5% were greater than 5 microns, 7.7% were 7 to 10 microns and 0.4% were greater than 10 microns. For coal particles, respective distributions were 57.6, 26.5, 11.2, 4.4 and 0.2%. Mineral dust particles were in significantly greater proportions in the two largest size categories relative to those of coal dust particles. Mass percent values for mineral dust particles were 16.3, 17.4, 52.1 and 14.2% for particles less than 5, 5 to 7, 7 to 10 and greater than 10 microns, respectively. Respective amounts for coal dust particles were 22.5, 11.3, 55.5 and 12.5%. Mineral and coal dust particles were significantly different for percent mass of particles over 5 and 7 microns and for fractions over 10 microns. Particle number distribution was greatest and mass percent lowest for smaller particles, and the reverse was noted for larger particles. The authors conclude that pathological lung changes in coal workers' pneumoconiosis are closely related to mass and content of retained respirable dust and that dust monitoring and hygiene standards should consider total as well as respirable dust concentrations.