Sexually transmitted infections may provoke a wide variety of rheumatic lesions. Disseminated N. gonorrhoeae infection leads to septic arthritis, which may be rapidly destructive but which responds promptly to appropriate antibiotic therapy. In contrast, both gonococcal and nongonococcal infections may lead to aseptic "reactive" arthritis or Reiter's syndrome. Inheritance of HLA B27 confers a relative risk of 30 to 50 times for the development of this condition. The demonstration of C. trachomatis antigen in joint material from a minority of patients suggests that direct interaction between microbial components and class I HLA antigens in the joint may be central to the pathogenesis of this disease. Arthralgia and arthritis occur in up to 50% of individuals in the prodrome of hepatitis B infection. Joint symptoms may be accompanied by urticarial or cutaneous vasculitic lesions, especially on the legs; both features resolve with the onset of jaundice. Hepatitis B infection is also a major cause of necrotizing vasculitis, which may or may not be associated with overt hepatitis. Seronegative arthritis, including Reiter's syndrome, psoriatic arthritis, and undifferentiated arthritis, a Sjögren's-like syndrome, vasculitis, and myopathies have been described in association with HIV infection. It is clear that synovitis occurs in those patients despite the fact that HIV is present in immune cells within the joint during inflammatory arthritis and that both antigen presentation and lymphocyte responsiveness within the joint are impaired. Nevertheless, synovitis may occur in the presence of marked CD4-positive lymphocyte depletion. Rheumatic syndromes, including arthralgia, inflammatory arthritis, and neuropathic arthritis, may occur during any stage of congenital or acquired syphilis. Syphilitic synovitis responds well to antibiotic therapy, but neuropathic lesions cannot be treated effectively. Septic arthritis has rarely been described as a complication of disseminated Mycoplasma or Urea-plasma infections, and joint lesions sometimes associated with erythema nodosum have also been reported in lymphogranuloma venereum and granuloma inguinale.