BACKGROUND: The safety of adding initial low-dose gentamicin to antistaphylococcal penicillins or vancomycin for treatment of suspected Staphylococcus aureus native valve endocarditis is unknown. This study evaluated the association between this practice and nephrotoxicity. METHODS: We performed a prospective cohort study of safety data from a randomized, controlled trial of therapy for S. aureus bacteremia and native valve infective endocarditis involving 236 patients from 44 hospitals in 4 countries. Patients either received standard therapy (antistaphylococcal penicillin or vancomycin) plus initial low-dose gentamicin (n=116) or received daptomycin monotherapy (n = 120). We measured renal adverse events and clinically significant decreased creatinine clearance in patients (1) in the original randomized study arms and (2) who received any initial low-dose gentamicin either, as a study medication or or=65 years and receipt of any initial low-dose gentamicin. CONCLUSIONS: Initial low-dose gentamicin as part of therapy for S. aureus bacteremia and native valve infective endocarditis is nephrotoxic and should not be used routinely, given the minimal existing data supporting its benefit.