Cement is sometimes applied to fix prostheses during arthroplasty. The cement can contain antibiotics for prophylaxis or treatment of joint infection. Gentamicin in cement is released gradually into its surroundings and can reach toxic concentrations in recovered blood. To determine gentamicin concentrations in patients with recovered blood and after blood reinfusion in patients who underwent joint replacement using gentamicin-loaded bone cement. This is a prospective observational study of 18 patients who underwent partial or total hip or knee replacement. Gentamicin-loaded bone cement was justified in all cases, and all patients were candidates for blood reinfusion after surgery. Gentamicin serum concentrations were measured by immunoassay. Serum concentrations were never higher than 1.5 µg/ml. The median concentration of gentamicin in recovered blood was 26.3 µg/ml. The maximum gentamicin dose by blood reinfusion could be 129 mg. The maximum gentamicin concentration in reinfused blood was 1.5 µg/ml. Gentamicin-loaded bone cement is safe, since drug levels did not reach toxic concentrations.