BACKGROUND & AIMS. Secondary loss of response is a frequent event occurring during biological therapy. The aim of this study was to assess loss of efficacy in patients with Crohn's disease treated with infliximab or adalimumab for a year. The secondary goals were to identify clinical or laboratory predictors of loss of response. METHODS. Sixty-one Crohn's disease patients achieved remission after the induction therapy and received regular maintenance therapy: 35 of them were on infliximab, 26 on adalimumab therapy. None of the patients treated with infliximab had received previous biological therapy, and 10 of the adalimumab-treated patients were naïve to biological therapy. The data of patients who relapsed and who remained in remission and also the characteristics of infliximab-treated patients and adalimumab-naïve patients were compared. Data were analyzed statistically. RESULTS. Remission was achieved in 70.5%; response was achieved in 29.5% of the patients after induction. Loss of response occurred in 22 of 61 patients after a year of therapy. The proportion of remission after induction was significantly lower in patients who lost response vs. those who remained in remission. More patients with sustained remission received immunosuppressive therapy before and during the biological therapy vs. those with loss of response. Loss of response was significantly more frequent and occurred earlier in adalimumab-naïve patients vs. infliximab-treated patients. CONCLUSION. The need for dose escalation should be calculated in the budget in the majority of patients, especially in adalimumab-treated patients.