<h3>Background</h3> The use of MTX for GVHD prophylaxis is associated with increased rates of oral mucositis (OM), delayed engraftment, hepatotoxicity and nephrotoxicity. Based on limited data, the EBMT and ELN working group recommend the use of FA-rescue to reduce MTX toxicity after alloHCT. We aimed to determine whether FA-rescue reduces the rate of MTX-induced toxicity in patients who receive post-transplant MTX for GVHD prophylaxis. <h3>Methods</h3> This is a double blind RCT conducted in 3 centers. We enrolled patients undergoing alloHCT from MSD or MUDs with a MAC regimen and PBSC grafts, and GVHD prophylaxis consisting CSA/MTX, plus ATG for MUDs. Patients were randomized to oral FA or placebo, stratified by center and conditioning intensity (standard <i>vs.</i> reduced toxicity MAC). FA administration started 24h after each MTX dose: 15 mg TID after MTX administration on day +1 and QID after MTX administration on days +3 and +6. The primary endpoint was the rate of grade 3-4 OM according to the WHO scale. A sample size of 58 subjects in each group was estimated to detect a difference in grade 3-4 OM of 50% <i>vs.</i> 25%. <h3>Results</h3> From 2/2016 through 7/2019, 52 patients with acute leukemia in CR (n = 45), MDS (n=6) or NHL in CR (n=1) were randomized to FA (n=28) or placebo (n=24). Patient characteristics were similar between groups (table). After enrolment of 52 patients, interim analysis revealed that the rate (46.6% <i>vs.</i> 45.8%, P=0.97) and duration (4 <i>vs.</i> 4 days) of grade 3-4 OM and the rate of OM of any severity (83.3% <i>vs.</i> 77.8%, P=0.65) were similar in both groups. Also, time to neutrophil and platelet engraftment, rates of febrile neutropenia and blood stream infections, VOD, need for opiates or TPN and time from HCT to discharge were not significantly different between groups. With a median follow-up of 9 months we did not observe any difference in rates of acute or chronic GVHD, DFS and OS. <h3>Conclusions</h3> The interim results, showing similar rates of grade 3-4 OM and secondary outcomes, do not support continuation of the study. FA-rescue following MTX GVHD prophylaxis does not seem to decrease regimen related toxicity and affect transplant outcomes.