Belumosudil was approved by US Food and Drug Administration in July 2021 for the treatment of relapsed/refractory chronic graft-versus-host disease (cGVHD) in patients aged ≥12 years after failure of at least 2 prior lines of therapy (LOTs). By comparing the treatment response pattern of patients between the Europe and US, this transportability analysis modelled belumosudil’s effect in European patients based on the observed effect in real-world US patients. This ROCKreal study used a targeted maximum likelihood estimation-based transportability analysis to compare treatment-response pattern in Europe and US to evaluate the potential efficacy of belumosudil compared with BAT for treatment of cGVHD in European patients after failure of 2 to 5 prior LOTs. This analysis generalized US ROCKreal study findings to European patients. Data was collected on 196 US and 222 European patients ≥12 years of age with cGVHD treated with 2 to 5 prior LOTs with visits between March 2015 and 2024 from 8 US and 32 European sites. Sustained steroid reduction is recognized as a clinically meaningful indicator of benefit in late-line cGVHD, consistent with prior work examining composite clinical benefit measures. The primary endpoint was the adjusted (causal) ratio in 6-month overall response rate (ORR), defined using a prespecified sequential algorithm incorporating NIH consensus criteria, physician assessment, and ≥50% corticosteroid taper without progression. Because outcomes were not collected for European patients, efficacy endpoints were transported using US outcome models. A supplementary endpoint, the adjusted (causal) difference in 6-month ORR, was also evaluated. At LOT initiation, median age (52.6 versus 57.6 years) and rate of severe cGVHD (40% versus 51%) were lower in Europe than the US; otherwise, populations were well-balanced. Efficacy was defined as ORR at 6 months post-LOT initiation when treated with belumosudil versus best available therapy (BAT). Using targeted maximum likelihood estimation, an outcome model was trained on US data. Treatment-specific responses were predicted using European patient characteristics, resulting in an estimated ORR of 37.6% when treated with belumosudil versus 26.3% with BAT (ORR ratio = 1.427 95% CI: [1.05, ∞); P-value: .03]). In line with positive efficacy results observed in previous real-world evidence studies in US and European patients, findings from this transportability analysis indicate that the treatment-response pattern are similar in Europe compared to US and cGVHD patients treated with belumosudil would have significantly more clinical benefit compared to BAT in Europe.