We evaluated the ability of both the conventional and high resolution computed tomography (CCT and HRCT, respectively) scans of the thorax to detect early silicosis in subjects exposed to silica dust in the mines and foundries of Québec for an average of 29 +/- 2 yr. The study was limited to subjects with chest radiograph (CR) of the International Labor Organization (ILO) Categories 0 or 1 as determined independently a priori. All subjects had a standard high-kilovoltage posteroanterior and lateral CR, a set of 10 to 15 1 cm collimation CCT scans, and a set of three to five 2 mm collimation HRCT scans in the upper, middle, and lower lung fields. For each CR and sets of CT scans, readings were done independently by four experienced readers. For small opacities of the lung parenchyma on CR, 32 of the 51 subjects were normal (Group A), six were indeterminate (Group B), and 13 were abnormal (Group C). By the combined readings of HRCT and CCT, 13 of the subjects (40%) in Group A were abnormal (p less than 0.001); four of the subjects in Group B were abnormal, and in Group C, one subject was normal, one indeterminate, and 11 (84%) abnormal. For confluence of small opacities, 48 of the 51 subjects were negative (Group 0), and three were positive (Group 1) on the CR. By the CT scan, 42 of the 48 subjects in Group 0 were negative, and the three subjects in Group 1 were positive; thus the CT scan added six positive cases with confluence of small opacities (six of 48, 12.5%).(ABSTRACT TRUNCATED AT 250 WORDS)