Routine cytopathologic examinations were performed at six-month intervals on sputum specimens from 628 former asbestos workers and 138 control patients. The occurrence of ferruginous bodies in sputa is found to increase as a logarithmic function of the length of occupational exposure to asbestos in workdays. No significant association is found between the occurrence of ferruginous bodies and the worker's age, smoking history, degree of cellular epithelial atypia, or time since last exposure. We conclude that the presence of ferruginous bodies in sputa is evidence of probable significant occupational exposure to asbestos dust. Their absence does not indicate lack of exposure. We can also conclude that routine cytopathology procedures are sufficient for the detection of ferruginous bodies in sputa.