Prospective, Multicenter Study on the Association Between Serum Ustekinumab Levels and Clinical, Biological and Endoscopic Remission in Patients With Crohn's Disease.

Inclusion Criteria: * Patients with an established diagnosis of CD according to standard criteria (clinical, analytical, endoscopic, radiological, and/or histological) with inflammatory activity, who begin treatment with ustekinumab for remission induction. Activity will be considered to be the presence of clinical activity (Harvey-Bradshaw index ≥5) and/or elevated fecal calprotectin (≥ 250μg/g; ≥100 in cases of surgically treated CD) and/or the presence of endoscopic activity (SES-CD ≥3). * Patients with a confirmed diagnosis of CD according to standard criteria (clinical, analytical, endoscopic, radiological, and/or histological) undergoing stable maintenance treatment with ustekinumab for at least 6 months. Exclusion Criteria: * Patients in whom ustekinumab is indicated for the prevention of post-surgical recurrence. * Patients in whom ustekinumab is indicated for the control of perianal disease. * Patients in whom ustekinumab is indicated for the control of extraintestinal manifestations. * Pregnant women. * Patients who are unable to understand the nature of the study, the procedures to be followed, or who are not able to sign an informed consent form.