Twenty-eight patients with rheumatoid pleural effusion were investigated to examine the frequency of HLA antigens as compared with 56 rheumatoid arthritis (RA) patients without this intrathoracic manifestation of RA and with 283 healthy controls. HLA-B8 was strongly associated with the presence of pleural effusion (PE) in RA patients. A high prevalence (71%) of B8/Dw3 was found among male RA patients of the PE group in whom the joint disease had begun at an age over 50 years and who also had besides pleuritis other intrathoracic manifestations of RA associated with high rheumatoid factor titres and low complement (C4) levels in sera. Actually, the HLA-B8 association was not seen in the rest of the PE group. The finding may be related to the heterogeneity of RA, a male subgroup of the disease being characterized by multiple intrathoracic manifestations and genetically associated with the large group of autoimmune disorders, such as SLE, characterized by high prevalences of HLA-B8 and D(R)3.