The aim of this observational prospective study was to compare the effect of fosfomycin tromethanol (FT) and carbapenems (meropenem or imipenem cilastatin) in the treatment of extended-spectrum beta-lactamase (ESBL)-producing Escherichia coli-related complicated lower urinary tract infection (CLUTI). Inclusion criteria were: patients who were aged >18 yr with dysuria or problems with frequency or urgency in passing urine; those with >20 leukocytes/mm³ in urine microscopy and culture-proven ESBL-producing carbapenem or FT-sensitive E. coli in the urine (>10⁵ cfu/mm³); no leukocytosis or fever; and who were treated with ft (oral 3 g sachet x 1 every other night, three times) or carbapenems between march 2005 and January 2006 in our outpatient clinic and hospital. A total of 47 CLUTI attacks in 47 patients (27 FT group, 20 carbapenem group) were observed prospectively. Clinical and microbiological success in the carbapenem and ft groups was similar (19/20 vs 21/27 and 16/20 vs 16/27 p>0.05). Drug acquisition costs were significantly lower in the FT group (p<0.001). Although it is not a randomized controlled study, these data show that ft may be a suitable, effective and cheap alternative in the treatment of ESBL-producing E. coli-related CLUTI.