The incidence of inflammatory joint diseases was estimated by using two patient series. Firstly, the total yearly incidence of all such diseases together was estimated in a population of 15 600 persons of 16 years of age or older. Secondly, this overall incidence was divided by the ratio of different diseases obtained from a larger series of patients. The incidence of all inflammatory joint diseases was 218/100 000/year, 182 in males and 250 in females. The incidence was highest in middle age and lowest in old age. The incidence of ill-defined arthritides was five times that of definite rheumatoid arthritis in the youngest age group but in the oldest their frequencies were equal. In the whole population, the proportion of ill-defined arthritides was 2/5, of definite RA 1/5, of HL-A B27 associated diseases 1/5, and of other diseases 1/5 of the total incidence of inflammatory joint diseases. Because the frequency of HL-A B27 in all patients surveyed was about 40%, only half of the patients with this antigen showed a clinical picture of ankylosing spondylitis, Reiter's disease, or reactive arthritis.

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.

Patients with mixed connective tissue disease (MCTD, n=32)

A comparison of clinical, immunological and HLA-D region antigen features was made between 22 patients with mixed connective tissue disease (MCTD) and 118 patients with systemic lupus erythematosus (SLE), scleroderma or primary Sjögren's syndrome. The MCTD patients had hypergammaglobulinemia more often than did those with SLE and scleroderma, but had less skin ulceration, serositis, nephritis, central nervous system disease and hypocomplementemia than the SLE patients. The frequencies of HLA-DR4 and its Dw4 subtype were significantly increased in MCTD as compared with both the other patient groups and healthy controls. Anti-RNP antibodies and the clinical characteristics together seem to illustrate a disease syndrome which is clinically and genetically distinct and fits with the prevailing concept of MCTD.