The study has the aim to investigate the efficacy and safety of etanercept in patients with active undifferentiated spondyloarthritis (uSpA) over the period of 520 weeks.

Inclusion Criteria:

1. Patients 18 to 65 years of age.
2. Proven undifferentiated spondyloarthritis according to the European Spondylarthropathy Study Group (ESSG) criteria who do not have ankylosing spondylitis, reactive arthritis, psoriasis and/or inflammatory bowel disease
3. Acute phase of disease with high disease activity the Bath Ankylosing Spondylitis Disease Activity Index (BASDAI) score ≥ 4 or a pain score ≥ 4 on a Numeric Rating Scale (NRS) at two occasions in 2 weeks
4. Understand, sign and date the written informed consent at the screening visit.
5. Sexually active women participating in the study must use a medically acceptable form of contraception until 6 month after the last injection of study medication. Medically acceptable forms of contraception include oral contraceptives, injectable or implantable methods, intrauterine devices, or properly used barrier contraception. Additionally, the use of condoms is suggested as an adjunct to the methods previously addressed to protect against sexually transmitted diseases and to provide additional protection against accidental pregnancy.
6. Sexually active men must agree to use a medically accepted form of contraception during the study until 6 month after the last injection of study medication.
7. Negative serum or urine pregnancy test taken at screen in all women except those surgically sterile or at least 1 year postmenopausal.
8. Able to self-administer injectable drug supplies or have a caregiver who will do so.
9. Able to store injectable test article at 2° to 8° C.

Exclusion Criteria:

1. Pregnancy/lactation
2. Previously exposure to murine or chimeric monoclonal antibodies
3. Receipt of any live (attenuated) vaccines within 4 weeks before screening visit
4. History of chronic or a recent serious infection
5. History of tuberculosis within the last 3 years
6. History of malignancy
7. Significant concurrent medical diseases including uncompensated congestive heart failure, myocardial infarction within 12 months, stable or unstable angina pectoris, uncontrolled hypertension, severe pulmonary disease, history of human immunodeficiency virus (HIV) infection, central nervous system demyelinating events suggestive of multiple sclerosis
8. Presence or history of confirmed blood dyscrasias
9. History of any viral hepatitis within 1 year prior screening or history of any drug-induced liver injury at any time prior to screening
10. Laboratory exclusions are: hemoglobin level < 8,5 mg/dl white blood cell count < 3.5 x 10^9/l platelet count < 125 x 10^9 /l creatinine level > 175 mcmol/l, liver enzymes > 1.5 times the upper limit of normal or alkaline phosphatase > 2 times the upper limit of normal.
11. Participation in trials of other investigational medications within 30 days of entering the study
12. Clinical examination showing significant abnormalities of clinical relevance
13. Concomitant medication with disease-modifying anti-rheumatic drugs (DMARDs) or corticosteroids
14. History or current evidence of abuse of "hard" drugs (eg., cocaine/heroine) or alcoholism