Studies have suggested associations between lower ratios of serum creatinine to cystatin C with both lower muscle mass and adverse clinical outcomes in multiple disease conditions. Identifying risk factors for mortality among patients with acute kidney injury (AKI) undergoing continuous kidney replacement therapy (CKRT) may improve assessment of prognosis. We sought to evaluate the association of creatinine–cystatin C ratio with outcomes in patients with AKI undergoing CKRT.