Prospective validation of the hematopoietic cell transplantation–comorbidity index (HCT-CI) using contemporary patients treated with hematopoietic cell transplantation (HCT) across the Unites States is necessary to confirm its widespread applicability. We performed a prospective observational study including all patients (8115 recipients of allogeneic and 11,652 recipients of autologous HCT) who underwent a first HCT that was reported to the Center for International Blood and Marrow Transplant Research between 2007 and 2009. In proportional hazards models, increased HCT-CI scores were independently associated with increases in hazard ratios for nonrelapse mortality (NRM) (P < .0001) and overall mortality (P < .0001) among recipients of allogeneic HCT. HCT-CI scores of ≥3 were uniformly associated with higher risks for outcomes in both allogeneic and autologous HCT and in all subgroups, regardless of diagnoses, age, and conditioning intensity. Recipients of allogeneic HCT with scores of 1 and 2 who were ages < 18 years or were treated with lower intensity conditioning regimens had similar outcomes compared with those with a score of 0. Higher risks for overall mortality, but not for NRM, were observed among recipients of autologous HCT with scores of 1 and 2 versus 0. Our results confirm the validity the HCT-CI in both allogeneic and autologous HCT. The index should be used as a valid standard-of-care health measure in counseling patients for HCT, in clinical trial design, and in adjusting outcome analyses.