The analysis of the clinical and HLA profiles of 99 patients with Reiter's syndrome is reported. Antigen HLA-B27, which has previously been firmly associated with Reiter's syndrome, predisposes patients to develop disease features which reflect articular involvement. The HLA haplotype A2, B27 was found to be at an elevated frequency in our Reiter's syndrome sample, and the latter two antigens are also associated with a general increase in disease severity. Conversely, antigen BW35 appears to be protective against certain features of the syndrome. Patients with certain antigenic profiles (namely A2 and A3 together with B27) tend to develop certain syndrome manifestations earlier in the course of the disease than those with other antigens.