• 180 patients suffering from active Crohn's disease with evidence of mucosal ulceration will be evaluated for improvement of disease activity (efficacy) when taking GLPG0634 or matching placebo once daily for 20 weeks in addition to their stable background treatment.
• During the course of the study, patients will also be examined for any side effects that may occur (safety and tolerability), and the amount of GLPG0634 present in the blood (Pharmacokinetics) as well as the effects of GLPG0634 on disease- and mechanism of action-related parameters in the blood and stool (Pharmacodynamics) will be determined. Also, the effects GLPG0634 administration on subjects' quality of life will be evaluated.

• 180 patients suffering from active Crohn's disease with evidence of mucosal ulceration will be evaluated when taking GLPG0634 or matching placebo once daily in addition to their stable background treatment. The population will include 50% anti-TNF naïve patients and 50% of subjects previously exposed to anti-TNF.
• The study will consist of 2 parts, with total treatment duration of 20 weeks. Randomisation in Part 1 will be stratified according to subject's previous anti-TNF exposure, C-reactive protein (CRP) level at Screening and oral corticosteroid use at Day -1. However, at Week 10, subjects will be re-randomized automatically and stratified according to the subject's clinical response (reduction of Crohn's Disease Activity Index (CDAI) of 100 points), previous anti-TNF exposure and corticosteroid use at Day -1 to receive GLPG0634 200 mg q.d., 100 mg q.d. doses, or matching placebo q.d. in a blinded fashion. In Part 2, all will continue the study until Week 20.
• As efficacy parameters, the ability to achieve clinical response or remission, endoscopic response & remission as well as mucosal healing with GLPG0634 given once daily compared to placebo will be evaluated after 10 weeks of treatment. In subjects who achieved clinical remission at Week 10, maintenance of the remission will be assessed during Part 2 of the study.
• During the course of the study, patients will also be examined for any side effects that may occur (safety and tolerability), and the amount of GLPG0634 present in the blood (Pharmacokinetics) as well as the effects of GLPG0634 on disease- and mechanism of action-related parameters in the blood and stool (Pharmacodynamics) will be determined. Also, the effects of different doses and dose regimens of GLPG0634 administration on subjects' quality of life will be evaluated.

Inclusion Criteria:

• Male or female subjects between 18 and 75 years
• Documented history of ileal, colonic, or ileocolonic CD
• CDAI score ≥ 220 to ≤ 450
• Evidence of active inflammation as demonstrated by endoscopic confirmation of active disease
• Subjects previously not exposed to anti-TNF treatment (TNF-naïve) or subjects previously exposed to anti-TNF therapy at a registered dose, that has been discontinued at least 8 weeks prior to Screening and deemed by the treating physician as a primary or secondary non-responder or intolerant (TNF-experienced)
• Continuation of concurrent treatment with oral steroids (≤30 mg prednisolone eq/day), mesalazine, olsalazine, CD-related antibiotics and probiotics at stable dose is allowed
• Previous exposure to immunomodulators is permitted, but must be discontinued
• Haematology and biochemistry lab parameters within predefined ranges as stated in the protocol

Exclusion Criteria:

• Diagnosis of indeterminate colitis, ulcerative colitis (UC), or clinical findings suggestive of UC
• Stoma, gastric or ileoanal pouch, procto- or total colectomy, symptomatic stenosis or obstructive strictures, history of bowel perforation, (suspected) abscess; actively draining fistulae
• Subject who has had surgical bowel resections within the past 6 months, short bowel syndrome or is receiving tube feeding, defined formula diets, or parenteral alimentation
• Subject with positive Clostridium difficile toxin stool assay or evidence of any other gastrointestinal infection
• Subject who has received non-permitted IBD therapies within specified timeframes, depending on the medication, as stated in the protocol
• Subject with a (previous history of) dysplasia of the gastrointestinal tract
• Concurrent gastro-intestinal malignancy or a history of cancer elsewhere
• History of lymphoproliferative disease
• Known active infection of any kind, current therapy for chronic infection or history of specific infections as stated in the protocol
• Subject who is pregnant, lactating or not willing to maintain highly effective birth control methods during the course of the study and 12 weeks thereafter