The clinical evaluation of antimicrobial agents for the treatment of urinary tract infections (UTIs) requires clinically useful categorization of patients plus concurrent laboratory diagnosis. Appropriate diagnostics include (1) proper collection techniques, (2) quantitative urine culture, (3) urinalysis to determine pyuria, and (4) radiological, urological and gynecological evaluation. Categorization of patients suffering from UTIs should take into consideration (1)the site of infection, (2) the clinical presentation, (3) the frequency of infections, and (4) the coexistence of complicating factors. UTIs in men and older women are mostly complicated. The characterization of a UTI as complicated or uncomplicated is important since in the former condition, multiresistant nosocomial microorganisms are frequently encountered. Experience with fleroxacin 200 or 400 mg once daily (o.d.) in > 1,000 patients with complicated UTIs in noncomparative and comparative trials with other quinolones and cephalosporins has been very promising, with overall clinical and bacteriological cure rates of 86-95 and 89-95%, respectively. As with other qumolones, relapses and superinfections were mostly attributed to Enterococcus, Staphylococcus and Pseudomonas spp. The reported results make oral o.d. fleroxacin appropriate as an alternative to parenteral antimicrobials or for intravenous-to-oral switch therapy in complicated UTIs.