Matthay MA, et al. - In this prospective, double-blind, multicentre, randomised, phase 2a trial of 60 eligible patients in 5 university medical centres in the USA, researchers evaluated the safety of administering mesenchymal stromal cells (MSCs) to patients with moderate to severe acute respiratory distress syndrome (ARDS). Via random assignment 2:1, participants were given either 10 × 10⁶/kg predicted bodyweight MSCs or placebo. The patients younger than 18 years, those with trauma or moderate to severe liver disease, and those who had received cancer treatment in the previous 2 years were excluded. The predefined MSC-related haemodynamic or respiratory adverse events were not evident in patients. In the MSC group, only 1 patient died within 24 h of MSC infusion, but death was judged to be probably unrelated. The hazard ratio for mortality at 28 days was 1·43 following adjustment for Acute Physiology and Chronic Health Evaluation III (APACHE III) score. Viability of MSCs ranged from 36% to 85%. Overall, one dose of intravenous MSCs was safe in this patient population.