Complex urinary tract infections are associated with many factors that increase the chance of bacterial infection while decreasing the effectiveness of treatment. Patients with complicated urinary tract infections usually have abnormalities in the structure and function of the urinary tract, patients have decreased resistance or increased virulence of bacteria and the emergence of drug-resistant bacteria, and the vast majority of patients with complicated urinary tract infections are male. Factors associated with complicated urinary tract infections include: abnormal urinary tract function and structure; male; pregnancy; old age; diabetes mellitus; immunosuppression; childhood urinary tract infections; recent antibiotic application; indwelling catheterization; urethral instrumentation; hospital-acquired infections; and clinical presentation beyond 1 week.