An investigation was conducted to examine the individual and combined effects of long term exposure to silica (14808607) and compressed air in a nonhuman primate model using cynomolgus monkeys. In a longitudinal study over 26 months of exposure, various parameters were examined using bronchoalveolar lavage (BAL) cytological and functional assessment of free lung cells, biochemical analyses of BAL fluid, lung function tests, radiological and pathohistological examinations as chief methods. The focus of this investigation was placed on the correlation of cellular constituents of BAL and quantitative analysis of CT scans of the lung. In all experimental groups BAL cell viability and BAL fluid recovery did not change during the exposure period. However, total cell counts indicated a biphasic profile, with a peak after 6 months followed by a decline and a final increase, starting after 18 months. The first peak of total cell counts was caused by increases of lymphocytes, macrophages, neutrophils and mast cells, in that chronological order. In contrast, the final increase of total BAL cell counts was mainly due to a rise of neutrophils. Mean CT densities were significantly augmented in both dust exposed groups. There was perfect correlation of CT densities, obtained after 27 months exposure, and BAL total cell counts obtained after 26 months.