The term urinary tract infection (UTI) encompasses a broad range of clinical entities that share one characteristic: a positive urine culture. Clinical manifestations and responses to therapy are diverse even when comparable numbers of a particular bacterial species are identified on urine culture. These guidelines include discussion of acute uncomplicated cystitis, acute uncomplicated pyelonephritis, and complicated infections of the urinary tract. It is proposed that the finding of ⩾103 cfu/mL of urine defines significant bacteriuria in acute uncomplicated cystitis, ⩾104 cfu/mL in acute uncomplicated pyelonephritis and UTI in men, and ⩾105 cfu/mL in complicated UTI. The preferred clinical study design is prospective, randomized, and controlled with an active agent. Ideally, treatment with antimicrobial agents should eradicate the infecting organism, bring about the resolution of clinical signs and symptoms, have few adverse effects, and prevent reinfection.