An in vitro study on the prevalence of resistance to major antibiotics in 1,545 Gram-negative aerobic bacilli isolated from intensive care unit patients was undertaken in 16 community and university hospitals in Belgium. A customized dry microtitre panel carrying 16 antimicrobials over an extended concentration range was used for susceptibility testing. The study revealed a widespread resistance to broad-spectrum penicillins, an alarmingly high incidence of resistance to aminoglycosides and a reduced activity of third generation cephalosporins especially in nosocomial pathogens, and a still relatively high susceptibility to imipenem and ciprofloxacin. Wide variations in percentage of resistance to different antibiotics were observed between hospitals, and these differences were not related to type of hospital or number of beds.