The sensitivity to sulfamethoxazole (SMZ), trimethoprim (TMP) and TMP/SMZ of urinary tract pathogens from hospital patients was studied during a period of 2 years in which the use of TMP/SMZ gradually increased, mainly replacing the use of sulfonamides alone. No significant trend could be observed towards a higher frequency of resistance to SMZ, TMP or TMP/SMZ. On the contrary, the frequency of fully SMZ-sensitive strains of E. coli, Klebsiella and Staph. aureus increased significantly, and the frequency of SMZ-resistant strains of Klebsiella decreased significantly. Variations in the frequency of synergy between TMP and SMZ are mentioned.