The prevalence of pneumoconiosis, chronic bronchitis, and impaired lung function was studied among those 1,000 foundry workers (response rate 93.1%) with the longest exposure time (minimum 4.2, mean 17, SD 9 years) from a representative sample of 20 foundries. Pneumoconiosis was diagnosed from 100 x 100 mm radiographs, and the false positives and false negatives were evaluated from normal-size radiographs from all those with a positive finding and a sample of those with a negative finding. Chronic bronchitis was studied by means of a translation of the MRC Short Questionnarie on Respiratory Symptoms. Forced vital capacity and forced expiratory volume in 1 s were measured with a Vitalograph Single Breath Wedge Spirometer, and the FEV % was calculated from these variables. The subjects were grouped according to smoking habits and dust exposure, which could be fairly well evaluated from measurements performed in connection with the health survey. All comparisons were made between different subcategories. The overall prevalence of pneumoconiosis was 3.8%, when allowance had been made for false positive and false negative findings. Most cases were mild. Chronic bronchitis occurred more frequently among those occupied in jobs classified as dusty. Smoking also strongly increased its prevalence; a combination of both exposures produced the strongest effect. The effect of smoking was also evident as an impairment of lung function; however, no such effect of dust exposure could be shown in this material. Since this was a prevalence study, the selective removal of workers from dusty jobs probably led to underestimates of all the health effects studied. In spite of the effect of selection excess bronchitis could be demonstrated in workers from dusty environments. Therefore effective dust control must be initiated not only with regard to silica dust but also with respect to total dust.