ABSTRACTIntroductionEarly post‐kidney transplant (KT) changes likely impact body composition, resulting in adverse post‐KT outcomes. We estimated post‐KT trajectories of computed tomography (CT)‐based muscle quantity/quality and tested whether they were associated with mortality and death‐censored graft loss (DCGL) among frail and nonfrail recipients.MethodsWe leveraged a cohort of 294 adult KT recipients (December 2008–February 2020) with CT measurements (muscle quantity: skeletal muscle index; muscle quality: skeletal muscle radiation attenuation). We used mixed linear regression models to estimate 3‐year post‐KT muscle quantity/quality trajectories. Cox proportional hazard models quantified the association between time‐varying pre‐/post‐KT muscle mass measurements and post‐KT mortality and DCGL.ResultsMuscle quantity (−2.4 cm2/m2/year) and quality (−1.3 HU/year) decreased during the first 2 years post‐KT and then remained constant in the third year post‐KT. Change in muscle quantity (pinteraction < 0.01) and quality (pinteraction = 0.01) differed by frailty in the first 2 years post‐KT; change in muscle quality (pinteraction < 0.01) differed by lower extremity impairment in the third year post‐KT. Among frail recipients, lower muscle quantity (per 10 cm2/m2) was associated with elevated mortality risk (aHR: 2.00, 95% CI: 1.08–3.70), but not among nonfrail recipients. Among older (≥65 years) recipients, lower muscle quantity was associated with increased DCGL risk (aHR: 2.70, 95% CI: 1.04–7.04), but not among younger recipients. Lower muscle quality (per 10 HU) was associated with elevated mortality (aHR: 2.23, 95% CI: 1.61–3.08) and DCGL (aHR: 1.90, 95% CI: 1.16–3.12) risk.ConclusionLower pre‐/post‐KT muscle quantity/quality were associated with higher risks of post‐KT adverse outcomes. Pre‐/post‐KT rehabilitation to improve muscle quantity/quality may be an effective clinical intervention to minimize risks of adverse post‐KT outcomes.