The incidence of trimethoprim resistance was correlated with changes in prescription behaviour in the Nottingham area from 1978-1985. The prevalence of trimethoprim resistance amongEnterobacteriaceaeisolated from patients with urinary tract infection rose from 5 % to 15 % of total strains examined. Strains resistant to trimethoprim but susceptible to sulfamethoxazole appeared from 1980 onward and represented 35 % of the total trimethoprim-resistant strains examined in 1985. Co-trimoxazole (trimethoprim + sulfamethoxazole) has been generally available for prescription in the United Kingdom since 1969, whereas trimethoprim alone was released in October 1979. By 1983, prescriptions in the form of trimethoprim alone accounted for approximately 50 % (in hospitals) and 15 % (in the community) of total trimethoprim usage in the Nottingham area. Although the introduction of trimethoprim alone seems to have had only a minor effect on overall resistance levels, it has greatly increased the proportion of trimethoprim-resistant strains which are susceptible to sulfamethoxazole. This was particularly evident in strains ofProteusspp., in which 52 % of the total trimethoprim-resistant strains were sulfamethoxazole-susceptible in 1985.