The selection of antimicrobial agents is guided by the use of formularies which often constrain prescribing options. There are several factors which influence the inclusion of specific agents. Two of the most important factors are microbial etiology of a disease and the incidence of antibiotic resistance. Various surveillance programs have highlighted the regional differences in antimicrobial susceptibility/resistance among various pathogens. This simple formula enables individual physicians, pharmacy and therapeutic committees and managed care formulary managers to harness local etiology and susceptibility information. In this paper the formula is applied to community- and hospital-acquired urinary tract infections.