This cross sectional study will evaluate a sample of 360 subjects with luminal Crohn's disease who are receiving infliximab in 15-20 Canadian infusion centres. The sample will be stratified by disease activity [mild = CDAI <150, moderate/severe = CDAI ≥150] and whether or not subjects are on concomitant immunosuppressive therapy. Thus 4 strata will be created consisting of 90 subjects each. Consent will be obtained prior to or at Visit 1, subjects will retrospectively complete a CDAI diary card for the preceding 7 days and blood work will be drawn prior to infliximab infusion. Four weeks thereafter at Visit 2, subjects will provide an additional blood sample to measure serum infliximab concentrations and CRP levels. Between Visits 1 and 3, subjects will complete daily CDAI diary entries. Visit 3 will take place at the next scheduled infusion, 8 weeks after Visit 1 (+/- 7 days ).