Complicated urinary tract infections are infections in the setting of structural or functional abnormalities of the genitourinary tract. They encompass a wide variety of clinical syndromes and anticipated outcomes. The infecting micro-organisms isolated are more varied and demonstrate a higher prevalence of antimicrobial resistance in complicated compared to uncomplicated urinary tract infections. The usual duration of therapy is 7 to 14 days, although comparative trials to define optimal treatment duration are lacking. Long term success of antimicrobial treatment is dependent upon whether or not the underlying genitourinary abnormality can be corrected. Treatment of complicated urinary tract infections will usually be successful and may be permanent if the underlying abnormality can be corrected. If the underlying abnormality cannot be corrected, failure rates of 50% at 4 to 6 weeks following therapy are expected. Antimicrobial agents are similar to those used to treat uncomplicated urinary tract infection. Certain agents, such as nitrofurantoin, should be avoided for individuals with renal failure. No specific agent or class of agents has consistently demonstrated greater therapeutic efficacy where the infecting organism is susceptible to the given agent.