Purpose: During the SARS-CoV-2 pandemic, national and international societies have recommended continuing biological agents in patients with immune-mediated inflammatory diseases (IMID) in the absence of SARS-CoV-2 symptoms. However, adherence to biological treatment might decrease, because these recommendations contradict patients’ beliefs. Especially an increased concern about side effects could have influenced the adherence to biological treatment during the first lockdown. The primary objective was to investigate the impact of the first SARS-CoV-2 lockdown on adherence to biological treatment in IMID patients. Patients and Methods: In this prospective cohort study, IMID patients who received a biological agent before and during the first SARS-CoV-2 lockdown (March 2020-June 2020) were included. Patients were excluded if they did not complete the medication adherence report scale-5 (MARS-5) questionnaire at ≥1 visit before the lockdown and ≥1 visit during the lockdown. Adherence to biological treatment was measured with the MARS-5 and Medication Possession Ratio (MPR). Results: We included 157 IMID patients. The percentage of adherent patients, defined as MARS-5 score >21, was significantly lower during the lockdown compared to the period before the lockdown (88.5% vs 84.1%, p<0.001). Additionally, the overall percentage of adherent patients during the lockdown based on the MPR ≥90% was significantly lower compared to adherence based upon the MARS-5 (65.1% vs 84.1%, p<0.001). Conclusion: This study showed that the first SARS-CoV-2 lockdown negatively impacts adherence to biological treatment in IMID patients. Therefore, treating physicians should be aware of this problem to minimize the potential harmful effects of non-adherence.