ABSTRACT: Frailty is a state characterized by diminished physiologic reserve and increased vulnerability to stress and adversely affects outcomes in older patients. We aimed to determine the relationship between pre-hematopoietic cell transplant (HCT) frailty and grades 3 to 4 nonhematologic toxicities (Common Terminology Criteria for Adverse Events, version 5.0) and mortality in HCT recipients within 1 year after HCT and also examined whether age at HCT moderated that association. In a prospective longitudinal study of 117 patients aged ? 40 years undergoing HCT, we performed formal pre-HCT geriatric assessments. Frailty was assessed using Fried's criteria. Post-HCT toxicities were abstracted through medical record reviews. The prevalence of pre-HCT frailty was 21% and was not different in younger (40 to 59 years) versus older (?60 years) HCT recipients. Overall, frail recipients (versus nonfrail) had a higher cumulative incidence of any grades 3 to 4 nonhematologic toxicity (86% [95% confidence interval {CI}, 62% to 100%] versus 70% [95% CI, 57% to 83%), P?=?.03) and more organ-specific grades 3 to 4 toxicities, such as non-neutropenic infections (38% [95% CI, 17% to 59%] versus 13% [95% CI, 6% to 20%], P < .01), nervous system disorders (19% [95% CI, 3% to 35%] versus 4% [95% CI, 0 to 8%], P?=?.02), and pneumonia (38% [95% CI, 17% to 59%] versus 10% [95% CI, 4% to 17%], P < .01). Frail recipients were 1.9-fold (95% CI, 1.1 to 3.4) more likely to develop any grades 3 to 4 toxicities (P?=?.03), 4-fold more likely to suffer non-neutropenic infections (95% CI, 1.4 to 11) and pneumonia (95% CI, 1.4 to 12; both P?=?.01), and 8.6-fold (95% CI, 1.6 to 45.3) more likely to suffer nervous system disorders (P?=?.01). Frail allogeneic HCT recipients also had a 3.1 times (95% CI, .9 to 9.7; P?=?.06) higher risk of overall mortality as compared with nonfrail allogeneic HCT recipients. The higher toxicity and mortality observed in frail allogeneic recipients needs to be monitored with high attention. Studies focusing on interventions to reduce frailty and manage morbidities are needed.