To compare the efficacy, safety and pharmacokinetics of ABT-874 to placebo in subjects who have moderately to severely active Crohn's Disease.

Inclusion Criteria:

• Diagnosis of Crohn's disease for greater than 4 months confirmed by endoscopy or radiologic evaluation.
• CDAI score of >= 220 and <= 450 at Week 0.
• Males and females >= 18 years and < 75 years of age at the Screening visit.
• Judged to be in generally good health as determined by the Investigator.

Exclusion Criteria:

• Current diagnosis of the colitis other than Crohn's disease.
• Symptomatic known strictures.
• Surgical bowel resections within the past 6 months or is planning any resection at any time point while enrolled in the study.
• Ostomy or ileoanal pouch. (Subjects with a previous ileo-rectal anastomosis are not excluded).
• Short bowel syndrome as determined by the investigator.
• Infection or risk factors for severe infections.
• Females who are pregnant or considering becoming pregnant during the study, or breast-feeding.