question: BACKGROUND: We designed this single-centre clinical trial to assess the potential benefits of N-Acetylcysteine (NAC) in patients with COVID19-associated acute respiratory distress syndrome (ARDS). METHODS: Ninety-two patients with mild-to-moderate COVID19-associated ARDS were allocated to the placebo (45-cases) or NAC groups (47-cases). Besides standard-of-care treatment, the patients received either intravenous NAC at a dose of 40 mg/kg/day or the placebo for three consecutive days. The efficacy outcomes were overall mortality over 28-day, clinical status on day 28, based on the WHO Master Protocol, the proportion of patients requiring mechanical ventilation, changes in ARDS-severity (based on the PaO2/FiO2 ratio), and Sequential Organ Failure Assessment (SOFA) scores 48 and 96 h after intervention, RESULTS: No differences were found in the 28-day mortality rate between the two groups (25.5% vs. 31.1% in the NAC and placebo groups, respectively). Although the distribution of the clinical status at day 28 shifted towards better outcomes in the NAC-treated group, it did not reach a statistical significance level (p value = 0.83). Similar results were achieved in terms of the proportion of patients who required invasive ventilator support (38.3% vs. 44.4%), the number of ventilator-free days (17.4 vs. 16.6), and median time of ICU and hospital stay. Results regarding the change in PaO2/FiO2 ratio and SOFA scores also showed no significant differences between the groups. CONCLUSIONS: Our pilot study did not support the potential benefits of intravenous NAC in treating patients with COVID-19-associated ARDS. More studies are needed to determine which COVID-19 patients benefit from the NAC administration. TRIAL REGISTRATION: The trial was registered at Clinicaltrials.gov (identifier code: IRCT20120215009014N355). Registration date: 2020-05-18.