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.