Chronic GVHD (cGVHD) is associated with mortality, disability and impaired quality of life. Understanding the role of comorbidity in patients with cGVHD is important both for prognostication and potentially for tailoring treatments based on mortality risks. In a prospective cohort study of patients with cGVHD (n=239), we examined the performance of two comorbidity scales, the Functional Comorbidity Index (FCI) and the Hematopoietic Cell Transplantation-specific Comorbidity Index (HCT-CI). Both scales detected a higher number of comorbidities at cGVHD cohort enrollment than pre-hematopoietic cell transplant (HCT) (P<0.001). Higher HCT-CI scores at the time of cGVHD cohort enrollment were associated with higher non-relapse mortality (HR: 1.21:1.04-1.42, P=0.01). For overall mortality, we detected an interaction with platelet count. Higher HCT-CI scores at enrollment were associated with an increased risk of overall mortality when the platelet count was ≤100 000/μL (HR: 2.01:1.20-3.35, P=0.01), but not when it was >100 000/μL (HR: 1.05:0.90-1.22, P=0.53). Comorbidity scoring may help better to predict survival outcomes in patients with cGVHD. Further studies to understand vulnerability unrelated to cGVHD activity in this patient population are needed.