A functional study involving VC, FEV 1.0 sec, RV, steady-state DLCO and fractional CO up- take at rest and, where possible, at two exercise levels (50 and 100 W), has been carried out on 724 male subjects exposed to asbestos dust inhalation. Subjects were divided into 5 groups according to UICC radiological classification of Cincinnati (1970): group 1 0/0–0/1, with asbestos exposure of less than 10 years; group II 0/0–0/1, with asbestos exposure of more than 10 years; group III 1/0–1/1, group IV 1/2–2/1–2/2 and group V 2/3–3/2–3/3. The following results were obtained: obstructive ventilatory insufficiency was observed in a percentage of cases variable between 11.2 and 24.2% in the different groups; restrictive ventilatory insufficiency was observed in 5.6% of subjects in group I, 10.1 % in group II, and 31.1–41.9–69.2%, respectively, in groups III, IV and V; impairment of gas exchange at rest and during exercise (reference to the lowest values obtained in 123 normal male subjects) was found in 4.6–6.4% of subjects in group I, 9.6–19.2% of subjects in group II and in a percentage of above 20% (till 100 %) in groups III, IV and V. Low values of DLCO in some cases of asbestos-exposed workers without radiological changes and without obstructive pulmonary disease suggest that the pulmonary damage induced by asbestos fibers may determine functional alterations of gas exchanges even in the preradiological stages