The rationale for a preformed combination of fosfomycin and amoxicillin is briefly discussed. These two antibacterial drugs block the peptidoglycal biosynthesis sequentially, thus exerting a true synergistic effect. On the other hand, there is no no synergism as far as toxicity is concerned. As the kinetics of the two molecules are different, further research on this topic is needed. This is especially true for renal failure patients