Background Ustekinumab (UST), an inhibitor of the p40 subunit of interleukins 12 and 23, is approved for the treatment of moderate and severe Crohn disease (CD) but a significant number of patients either partially respond or do not benefit at all. Therapeutic drug monitoring can be used to optimize the efficacy of biological drugs by ensuring adequate exposure to these drugs. The aim of our research was to identify the relationship between serum concentrations of UST during the induction and biochemical remission at week 16 in patients with CD. Methods All CD patients treated with UST included in a local database were identified. Only patients with a fecal calprotectin (FCP) > 250 mg/g were selected. All patients were treated with an initial intravenous induction therapy of UST, followed by subcutaneous maintenance therapy. Disease activity was assessed retrospectively by clinical (Harvey–Bradshaw Index [HBI]) and biochemical parameters (FCP; C-reactive protein (CRP) and Albumin) at Week 0, Week 4, week 8 and week 16. UST trough levels were measured at 4, 8 and 16 weeks using a commercially available validated enzyme-linked immunosorbent assay (ELISA). Main outcome was biochemical remission, defined by a decreased of calprotectin levels more than 80% or an absolute value less than 200 mg/g at week 16. Results Seventy-six patients were included. Patients’ Characteristics at baseline are listed in table 1. Median [IQR] HBI was 5,5 [3-8] at the beginning of treatment. Evolution of biochemical parameters throughout induction is showed in table 2. At the end of the induction phase (week 16), HBI, CRP and FCP decreased significantly from baseline. At this moment HBI was less than 2 in 51% of patients. FCP was available in 63 patients. Biochemical remission was observed in 26/63 (41,3%) patients. Median [IQR] UST serum concentrations were 20,9 μg/mL [13,7–26,3] at w4, 10,1 μg/mL [3,5–7,4] at w8 and 4,1 μg/mL [1,4–3,9] at w16. Quartile analysis demonstrated that 100% and 66% with an UST serum concentration in the highest quartile [Q4], at w4 and w8, achieved a biochemical remission by w16 [p = 0.05 and p <0,02 respectively]. The area under the receiving operating characteristic curve (AUROC) of UST levels to predict biochemical remission at week 4 and week 8 were 0,84 CI95% (0,62-1) and 0,74 CI95% (0,57-0,91) respectively. Conclusion Serum concentrations of UST at week 4 and 8 after intravenous infusion could be used to stratify patients according to the probability of achieving remission. This measurements during induction could be used to optimize treatment of CD.