Purpose: Frailty, a multifactorial biological syndrome characterized by a cumulative dysregulation of physiological processes, is associated with changes in pharmacokinetics and pharmacodynamics. The aim of this study was to quantify the effect of frailty on glomerular filtration of drugs, using the probe drug gentamicin. Methods: Gentamicin concentrations and clinical data including the Reported Edmonton Frail Scale score were pooled from two prospective observational inpatient studies, one on prophylactic gentamicin for urologic surgery and one on therapeutic gentamicin for the empiric treatment of sepsis. Population pharmacokinetic modeling was performed using non-linear mixed effects modeling (NONMEM program) to determine the impact of frailty on gentamicin clearance. Results: A one-compartment linear pharmacokinetic model best described the data and the addition of frailty to the model reduced the random variability in gentamicin clearance by 12 after adjustment for renal function (estimated creatinine clearance using lean body weight) and lean body weight. Frail patients had an approximately 12 lower (bootstrapping results: 14 median) gentamicin clearance than non-frail patients (calculated as a fractional effect of frailty). Conclusions: Frailty may independently predict reduced clearance of gentamicin in older patients. Frailty could be considered in the development of dosing guidelines for drugs that undergo significant excretion through glomerular filtration.