If treatment with Adalimumab fails (despite optimal dose and interval) and the treating physician therefore decided to switch to infliximab, the patient may be enrolled in this observational study. At regular intervals (every Remicade), the patient will be clinically re-evaluated. The disease activity score will be calculated: Crohn's disease activity index (CDAI). At regular intervals, the results of interim blood tests will be documented (3x). The succession will be 1 year. At week 10, 26 and 52, additional serum samples will be taken for determination of antibodies against Adalimumab and Infliximab. The serum levels of Adalimumab (week 0) and Infliximab (week 10, 26 and 52) will be determined.