There are 8 cement works in Morocco employing 3 600 people and producing 8 million tons annually. The aim of our study is to determine the prevalence of respiratory symptoms and to propose some preventive measures. The study involved a group of workers of whom 280 were exposed to cement dust (who were further subdivided into three categories according to the intensity of exposure) and 73 who were not. It included a medical history with a standardised questionnaire, a clinical examination and spirometry. 65% of those exposed had clinical symptoms as opposed to 34.2% of the non-exposed. Cough, expectoration rhinitis, chronic bronchitis and asthma were significantly more common with incidences of 56.1%, 52.5%, 49.3%, 29.3% and 14.3% respectively in the exposed against 19.2%, 24.6%, 26%, 9.6% and 6.8% in the non-exposed. Among the exposed the prevalence of chronic bronchitis increased significantly with the degree of exposure, from 11.4% in category 1 to 41.6% in category 3. Age did not seem to influence the development of chronic bronchitis but on the other hand a smoking history of more than 10 years did. Exposure is responsible for the development of respiratory problems because among non smokers the exposed (55.6% are more symptomatic than the non-exposed (13.3%). Tobacco smoking potentiates the effects of occupational airborne contamination because exposed smokers (73.6%) and ex-smokers (67.8%) have more respiratory problems than exposed non-smokers (55.6%). The incidence of abnormalities of respiratory function is significantly higher in the exposed than in the non-exposed (32.5% against 13.7%). Among the exposed workers presenting disorders of ventilatory function 72.5% have no more than a disorder of the small airways or a slight deficit. Among the exposed the prevalence of impaired respiratory function is greater in smokers and ex-smokers (47.9% than in non-smokers (10.4%). Tobacco smoking augments the impairment of respiratory function. Prevention depends therefore on a programme of technical (collective and individual) and medical protection of the workforce.