HLA phenotypes were determined in 109 patients with rheumatic fever (RF), 48 patients with Yersinia arthritis (YA), 86 patients with chronic rheumatic heart disease (RHD), and 326 controls. There was an increased frequency of Bw35 in RF as compared to controls (Pc less than 0.01), while B18 was more common in patients with acute carditis than in those without (P less than 0.02). HLA frequencies in RHD did not differ significantly from those in controls. A significant correlation between B27 and YA was observed (Pc less than 0.001). Carditis or iritis occurred in 10 of 31 B27 positive YA patients but in none of 17 B27 negative patients. Eleven of 31 B27 carriers had signs of urological inflammation vs one of 17 B27 negative patients. In the B27 positive YA group, there were three men with previous ankylosing spondylitis and one with Reiter's syndrome (RS). Also, four patients developed RS during Yersinia infection. This simultaneous occurrence of three B27 positive rheumatic diseases suggests that a patient with one "B27 positive rheumatic disease" is more susceptible to other diseases or symptoms known to be associated with the B27 antigen.