Inclusion Criteria:

1. definite classification of the arthritis as ReA enteric ReA is defined as an arthritis, which occurs within 4 weeks after a preceding symptomatic infection of the gut with enteric bacteria such as yersinia, salmonella, campylobacter jejuni, shigella. If no symptomatic preceding infection can be remembered the triggering enterobacterium has to be clearly identified by serology or stool culture. Other causes for a diarrhea like for example inflammatory bowel disease have to be eliminated.

   urogenital (chlamydia-triggered) ReA is defined as an arthritis, which occurs within 4 weeks after a symptomatic urogenital infection or an infection of the upper airways or if chlamydia can be clearly identified be serology or direct proof.

2. disease duration > 12 months
3. age 18 to 70 years
4. active arthritis in at least one joint
5. constant demand of NSAIDs
6. intensity of pain > 4 on a visual analogue scale (VAS; 0 to 10)
7. patients are allowed to have been treated with so-called conventional therapy (Sulphasalazine, Methotrexate etc.) or steroids i.a. before, but they have to be stopped 4 weeks before enrolled into the trial
8. able to self-administer s.c. injections or have a caregiver who will do so
9. women of child bearing potential must have a negative pregnancy test at study baseline and use an adequate, effective method of contraception (such as implants, injectables, combined oral contraceptives, some IUDs, sexual abstinence, vasectomised partner) for a duration of 6 months after stop of therapy. Sexual active men must use an accepted method of contraception for a duration of 6 months after stop of therapy.
10. reading a normal chest/ lung x-ray, negative Mendel-Mantoux-skin test (10,0 TE) (both not older than 4 weeks). If Mendel-Mantoux-skin test is positive and / or there are hints for a healed up tuberculosis in the chest x-ray (latent tuberculosis) and the patient shall receive infliximab or etanercept an additional therapy with isoniazid 300 mg daily starting 4 weeks before first administration of infliximab or etanercept has to be given.
11. signed informed consent

Exclusion Criteria:

1. female subjects who are pregnant or breast-feeding
2. previous treatment with cytokines or anti-cytokines (biological agents)
3. severe infections within the last 3 months
4. history of opportunistic infections within the last 2 months (herpes zoster, cytomegaly virus-, pneumocystis carinii-infection)
5. HIV-infection
6. history of malignancy
7. receipt of any live (attenuated) vaccines within last 30 days before screening visit
8. previous diagnosis or signs of demyelinating diseases
9. history of uncontrolled diabetes, unstable ischemic heart disease, active inflammatory bowel disease, active peptic ulcer disease, recent stroke, ongoing congestive heart failure, and any other condition which, in the opinion of the investigator, would put the subject at risk by participation in the protocol.
10. history of cytopenia
11. laboratory exclusions are: hemoglobin level < 8,5 g/dl, white blood cell count < 3.5 x109/l, platelet count < 125 x 109 /l, creatinine level > 175 µmol/ liver enzymes > 1,5, alkaline phosphatase >2 times the upper limit of normal, Quick > 50.
12. clinical examination showing significant abnormalities of clinical relevance
13. participation in trials of other investigational medications within 30 days of entering the study
14. history or current evidence of abuse of ”hard” drugs (e.g. cocaine/heroine)
15. current medication with 7,5 mg or more Prednisolon daily