Pulmonary tissue obtained at thoracotomy or autopsy from 5 kaolin workers with complicated pneumoconiosis was studied by optical light and scanning electron microscopy. Premortem or preoperative chest roentgenograms demonstrated small irregular shadows and large opacities typical of kaolin pneumoconiosis. On gross examination, there were firm, grey-brown nodules and masses in the parenchyma and in the hilar lymph nodes. Histologically, there was extensive pulmonary kaolinite deposition associated with formation of peribronchiolar macules and nodules. The latter were comprised of kaolinite aggregates traversed by bands of fibrous tissue rather than dense whorled collagen, as seen in silicosis. Crystallographic studies confirmed the presence of kaolinite in the lungs, but silica was not demonstrable by either analytical scanning electron microscopy or X-ray diffractometry. These findings illustrate the pathology of human kaolin pneumoconiosis, confirm the fibrogenic potential of kaolinite, and emphasize differences in pulmonary responses to kaolinite and to silica.