The narrow range of therapeutic serum concentrations of aminoglycosides and vancomycin and the great variations in their pharmacokinetics from one person to another makes it important to monitor patients at risk that are treated with them. The technique of customizing dosages based on the population pharmacokinetics enables an effective treatment to be rapidly established from a few measurements of serum concentrations. Aminoglycosides may be given as single daily doses because of their concentration-dependent activity. The high peak concentration produces an enhanced, prolonged bactericidal activity, while a low residual concentration reduces the risk of toxicity. The effect of vancomycin is time-dependent. Giving it by continuous i.v. infusion maximizes the time during which the serum antibiotic concentration is effective but non-toxic. Monitoring serum concentrations can help reduce health care costs. But medical training in pharmacokinetics is needed for the optimal use of these therapeutic tools.