Abstract The sensitivity to sulphamethizole, sulphamethoxazole (SM), trimethoprim (TMP) and the combination SM/TMP of freshly isolated urinary tract pathogens from hospital patients was studied. Out of 798 strains tested primarily, 47% were found to be fully sensitive to sulphamethizole (tablet method), and 81% fully sensitive to the SM/TMP combination (disk method). Out of 423 strains with reduced sensitivity to sulphamethizole, 65% were fully sensitive to SM/TMP. Out of 391 strains tested secondarily (disk method), 39% were fully sensitive to SM, 68% fully sensitive to TMP, and 91% to SM/TMP. Of 283 strains presenting a reduced sensitivity to SM, 85% were fully sensitive to SM/TMP. In both series the isolated Streptococcus faecalis strains were highly resistant to sulphamethizole and SM, but presented a very high degree of sensitivity to TMP and SM/TMP. In contrast, nearly all isolated Pseudomonas aeruginosa strains were found to be resistant both to sulphonamides, TMP and SM/TMP. Of the 18 Staphylococcus aureus strains tested secondarily, 10 out of 12 multiresistant strains were fully sensitive to TMP. Synergy between SM and TMP could be demonstrated in 61% of the 391 strains tested secondarily. The synergic effect depended on the degree of sensitivity to SM.