A national multicenter study was carried out during the first semester of 1991 to confirm the in vitro activity of fosfomycin trometamol (FT) against a range of bacterial strains associated with UTI in out-patients. MICs of FT were determined by nutrient broth dilution method for an adjusted sample of 273 most representative urinary isolates. Concentration values of 8 and 64 mg/l required to inhibit respectively 50 and 90 % of strains were widely lower than mean urinary levels achieved within the 0-48 h interval after a single oral dose of 3 g FT. Species with MICs higher than 256 mg/l (St. saprophyticus, M. morganii, Providencia spp. and some P. aeruginosa) represent less than 5 % of strains isolated from UTI in out-patients. The spectrum of activity of fosfomycin trometamol actually embraces most of the common urinary pathogens isolated from UTI in out-patients.