Background: Blood or bone marrow transplantation (BMT) survivors with frailty are at a higher risk of subsequent mortality. Longitudinal trends in the frailty state are not known and could help identify vulnerable subpopulations at risk of subsequent adverse events. Methods: This study included a cohort of 470 autologous and allogeneic BMT recipients who had survived ≥2 years after BMT and completed a baseline questionnaire (t1) at a median of 7.3 years after BMT and a follow-up questionnaire (t2) 13.2 years after t1. The main outcome was change in frailty state between t1 and t2. Frailty phenotype was defined as exhibiting ≥3 of the following characteristics: clinically underweight, exhaustion, low energy expenditure, slow walking speed, and muscle weakness. The following categories of change in frailty state were evaluated: worsened, improved, and stable. Results: Of the 470 participants, 36.4% were aged ≥60 years at t1, and 50.6% were men. The prevalence of frailty increased from 4.8% at t1 to 9.6% at t2. Worsening was observed in 18.8% of patients, and improvement was reported in 9.7%. Pre-BMT exposure to vincristine (odds ratio [OR], 2.1; 95% CI, 1.3-3.39) was associated with worsening. Female sex (OR, 1.5; 95% CI, 0.93-2.4) was associated with a trend toward worsening. Pre-BMT exposure to vincristine (OR, 2.79; 95% CI, 1.44-5.43), a history of chronic graft-versus-host disease (OR, 2.58; 95% CI, 1.2-5.5), and grade 3 and 4 chronic health conditions at t1 (OR, 2.1; 95% CI, 1.08-4.33) were associated with frailty at t2. Conclusions: In a cohort of BMT survivors who were followed longitudinally for a median of 20.6 years from BMT, the frailty status worsened for approximately20% over a 13-year timespan. BMT survivors who are at risk for worsening frailty could benefit from targeted interventions.