We assessed the rates of antimicrobial resistance between 1990 and 1993 in intensive care units in the United States. A standardized microtiter minimal inhibitory concentration panel was used to test approximately 100 consecutive gram-negative aerobic isolates that were recovered primarily from blood, wounds, urine, and pulmonary sites in patients treated in each of 396 intensive care units in 45 states. Amikacin and imipenem were the agents most active against the 33,869 nonduplicate isolates (those recovered only once) tested. Resistance of aerobic gram-negative bacilli to third-generation cephalosporins was found to be an emerging problem. Increases in rates of resistance to ceftazidime among isolates of Klebsiella pneumoniae (from 3.6% to 14.4%; P << .01) and Enterobacter species (from 30.8% to 38.3%; P = .0004) were noted from 1990 to 1993; rates of resistance among Pseudomonas aeruginosa isolates remained stable. Ceftazidime-resistant bacteria were frequently resistant to aminoglycosides and ciprofloxacin. Risk factors for ceftazidime resistance included the number of beds in the hospital, the teaching status of the hospital, and specific body sites from which the isolates were recovered.