Background Feather duvet lung (FDL) is a rare subgroup of bird fancier's lung (BFL). We were interested in determining antigen-specific IgG antibodies in patients with FDL and comparing them with those with BFL. Material and methods Specific IgG antibodies against goose and duck feathers, analysed with an automated fluorimetric enzyme immunoassay, were measured in healthy subjects (group A, n = 30), in patients with FDL (group B, n = 10) and with BFL (group C, n = 35); typical specific IgG antibodies of BFL in groups B and C. Results An optimal threshold value for antibodies against goose or duck feathers to differentiate patients with either BFL or FDL from healthy subjects was determined at 10·85 mg L^-1 for goose feathers and at 8·81 mg L^-1 for duck feathers, respectively. Specific IgG antibodies against goose feathers were significantly higher in group B compared with group C. A ratio of specific IgG antibodies against goose feathers and budgerigar antigens with a threshold value of 0·91 could discriminate between patients with FDL and BFL with a specificity of 97% and a sensitivity of 90%. Conclusions We were able to demonstrate the significant difference in IgG antibodies in patients with FDL and BFL and their contribution to discriminate between these similar kinds of extrinsic allergic alveolitis.