OBJECTIVE: To characterize antimicrobial resistance patterns to amikacin (AN) and gentamicin (GM) among Gram-negative bloodstream isolates and to determine the possible relationship between use of AN and GM and the occurrence of antibiotic resistance during a 6-year period. METHODS: Standard media and techniques of isolation and identification were used. Antimicrobial susceptibility testing was performed with the disk diffusion method and API rapid ATB E strips. Data on consumption of aminoglycosides were collected by the central hospital pharmacy and were expressed as daily defined doses. RESULTS: One thousand nine hundred and four bloodstream isolates were tested for AN and GM susceptibility between 1989 and 1994. Activities of AN and GM remained high during the study period against most isolates of Gram-negative bacteria. No relationship could be observed between the use of AN/GM and the rate of AN/GM resistance. Nosocomial Gram-negative bloodstream isolates showed a higher degree of resistance towards both AN (3.9% of all nosocomial isolates) and GM (7.9%) than community-acquired isolates (1.8% toward AN and 3.1% towards GM, respectively). There was a significant increase (P=0.004) in the risk of GM resistance in patients with nosocomial Gram-negative bacteremia detected more than 14 days after admission. The proportion of GM-susceptible Pseudomonas aeruginosa isolates decreased linearly from 97% for infections acquired between day 3 and day 10 following admission to 80% for bacteremia developing 30 days or more after admission (P=0.008). CONCLUSIONS: AN and GM remain highly active antimicrobial drugs for treatment of GNB in times of growing resistance to cephalosporins and fluoroquinolones.