<p>Idiopathic pneumonia syndrome (IPS) is a serious complication of allogeneic hematopoietic stem cell transplantation (HSCT) and has a poor prognosis. Although IPS is often treated with steroids, the disease can become resistant to or dependent on steroid treatment, and there is no effective cure for patients with refractory or steroid-dependent IPS. This multicenter, open-label, single-arm, phase II clinical trial investigated the efficacy and safety of HLC-001 (allogeneic umbilical cord-derived mesenchymal stromal cells) in patients with progressive steroid-dependent or refractory IPS after HSCT. Seven male patients (all male; mean age: 43.3&#xa0;years) received HLC-001 and three completed the trial. The survival rate at day 56 (primary endpoint) was 71.4% (5/7 patients; 95% confidence interval: 29.0%–96.3%) and was sustained at day 100, suggesting that HLC-001 was more effective than previously reported treatment. Three of the five patients with ≥ 100&#xa0;days of follow-up died. Five patients experienced at least one adverse drug reaction, none of which were serious. These findings indicate that HLC-001 was potentially effective and generally well tolerated in Japanese patients with steroid-dependent or refractory IPS after HSCT. Given there is no effective cure for steroid-dependent or refractory IPS, HLC-001 may be a promising treatment option and further clinical evaluation is warranted.</p><p>Trial registration: Japan Registry of Clinical Trials identifier: jRCT2063220014.</p>