We studied 24 patients with bird fancier’s lung (proven by serological, clinical and radiological tests) in order to record the severity and pattern of defects in CO transfer. We measured dynamic and static lung volumes (FVC, FEV1, FRC, RV), lung mechanics (Raw, CL), CO transfer factors (TL, TM, VC), CO transfer coefficients (TL/VL, TM/VL, VC/VL) as well as blood gases ( $$Pa_{O_2 } , Pa_{CO_2 } $$ ). The only significant signs of restrictive function loss in this group were a reduced FRC and a reduced specific compliance. The impairment of CO transfer is primarily attributed to a reduced gas-exchange surface area rather than to thickening of the alveolo-capillary membrane. The reduction in CO transfer correlated with the duration of antigen exposure. There was evidence of overfilling of pulmonary capillaries. As a result of the changes in CO transfer, the resting arterial oxygen tension was reduced.