We investigated the relationship between the amount of ceftriaxone used in our hospital and the evolution of the rate of resistance among Enterobacter species isolates. We reviewed all positive microbiological reports for Enterobacter species and the pharmacy records for the ceftriaxone consumption during four semesters over 4 consecutive years. The resistance to ceftriaxone among Enterobacter species rose from 10 to 27% while the amount of ceftriaxone used almost trebled during the study period. In order to investigate the relationship between the use of an antibiotic and the emergence of resistance to it, we studied the development of ceftriaxone resistance in Enterobacter species and the use of this drug at the Centre Hospitalier Universitaire Vaudois (CHUV) each year between 1983 and 1986. The consumption of ceftriaxone more than doubled between 1983 and 1984, and since then has increased only slowly. Within 2 years the rate of Enterobacter species resistance to ceftriaxone has jumped from about 10% between 1983 and 1985 to 27% in 1986. As no particular modification in the preventive methods of hospital hygiene and no epidemics occurred during the period of our study, we conclude that the increase in the prescribing of ceftriaxone probably played a central role in the increased number of resistant strains of Enterobacter species.