A study was conducted on the chronic health effects of diesel exhaust on the respiratory system of diesel bus mechanics. The study population included 283 male workers who were evaluated with regard to questions concerning the relationship between diesel exposure and chronic morbidity (respiratory symptoms, chest radiographic observations, pulmonary function). Workers with long and short tenure were compared. In addition, the study population was assessed for increased chronic morbidity compared to an unexposed "blue collar" group. Each worker was evaluated by the Medical Research Council respiratory symptom questionnaire, chest x- ray examination, and spirometry. Alcohol and smoking histories were also taken. The mean tenure of the study group was 9 years, some of whom had previously worked in a foundry (10 percent) or had asbestos exposure (10 percent), but mostly for 5 years or less. Approximately 25 percent of the workers reported chronic cough and phlegm, with no correlation to tenure. The prevalence of wheezing was 14 percent and there were seven workers (2.5 percent) with pneumoconiosis of category one. Reduction of pulmonary function and severity of symptoms were consistently related only for dyspnea and wheezing. In comparison with the "blue collar" group, the study population had an increased prevalence of cough, phlegm, and wheezing, but with no association to tenure. The authors conclude that the diesel exhaust may cause some chronic effects in the form of cough, phlegm, and wheezing and a tenure associated decrease in pulmonary function.