We have compared the frequency of HLA antigens in groups of patients with rheumatoid arthritis (RA) who had pulmonary manifestations, other extra-articular features or articular disease alone. There was a significantly increased frequency of the antigen HLA-B40 in patients with lung involvement compared to other forms of extra-articular RA (relative risk 40.54), and when compared to the total RA group (relative risk 4.94). In contrast there was little variation in the frequency of HLA-DR4, which was significantly increased in all patient groups. These data suggest that HLA-B40 is a marker of increased susceptibility to lung disease in patients with rheumatoid arthritis.