A study of the respiratory health of workers who produce man-made vitreous fibers (MMVF) included 1,028 men (median length of employment, 18 yr) in 7 fibrous glass and mineral wool plants. Health assessment was by respiratory questionnaire, ILO 1980 Classification chest radiograph readings, and pulmonary function testing. Results from an earlier environmental survey were combined with job histories to derive individual exposure estimates. The study population was found to be generally healthy, with respiratory symptoms not related to the fiber exposure and no detected adverse lung function consequences of that exposure. Radiograph readings revealed low category profusion of small opacities: of 941, 63 (7%) in category 0/1, 25 (3%) in 1/0, and 6 in 1/1, with none in higher categories. Prevalence of small opacities increased with age and cigarette smoking. A greater prevalence of small opacities was found among current smokers in the two plants producing both ordinary and fine fibers (for profusion ⩾ 1/0, 1...