• Immunoprotein deposition has been demonstrated in the synovium of patients with Reiter's syndrome. Because of this fact and the controversy regarding the nature and cause of the conjunctivitis in Reiter's syndrome, conjunctival biopsy specimens were taken from eight patients with classic Reiter's syndrome. These specimens were examined histopathologically, immunopathologically, and ultrastructurally. Our findings indicated that perivasculitis and vasculitis were present, preponderantly with elements of the cell mediated immune system. Genetically susceptible persons (namely, those with positive HLA-B27 antigen) exposed to an agent (Shigella or Chlamydia) may trigger an immune response in which vasculitis and perivasculitis plays a predominant role.