The isolation and immunological studies reviewed here appear to have proved that there is an association with Chlamydia trachomatis in reactive arthritis, and particularly in Reiter's disease. This condition, which occurs predominantly in young men, usually is precipitated by infection, sometimes enteritic but at present more commonly sexually transmitted. In the case of nongonococcal urethritis, C. trachomatis has been shown to be an agent that can trigger Reiter's disease. Patients with HLA B27 antigen are especially at risk. Clinical observations suggest that long-term tetracycline treatment may have a beneficial effect.