Ninety-four patients with rheumatoid arthritis who possessed one or more of the HLA DR alloantigens 2, 3, or 4 were studied to investigate the genetic influence on disease severity and prognosis. In those with a disease duration of less than 10 years radiological damage was less in patients with DR2 than in those without this antigen. When current joint scores were compared, patients with this antigen had less evidence of disease than patients with DR3 or 4, DR3 patients having the highest scores. The presence of nodules and Sjögren's syndrome were less common in the DR2 patients. Variability in response to disease modifying drugs according to the patient's HLA DR antigen status may explain these differences. It is concluded, however, that possession of HLA DR2 may be an indicator of good prognosis in patients with rheumatoid arthritis.