Susceptibility data were collected for 6243 gram-negative isolates from 29 European ICUs participating in the Meropenem Yearly Susceptibility Test Information Collection (MYSTIC) Programme (1997-2000). The most commonly isolated bacteria were Pseudomonas aeruginosa (22.5%), Escherichia coli (19.8%), Klebsiella pneumoniae (10.4%), and Enterobacter cloacae (7.7%). The incidence of extended-spectrum beta-lactamase-producers was higher in Turkish, Russian and Italian ICUs (27.9-39.6%) than in other countries (2.5-10.8%). The frequency of AmpC-cephalosporin hyper-producers was 16.8-55.4%. Meropenem was more active against Proteus mirabilis than imipenem (99.0% versus 88.8% susceptibility, respectively). Against Acinetobacter baumannii, meropenem (79.6% susceptible) and imipenem (82.2%) were more active than comparators (34.3-51.6%). Meropenem and imipenem exhibited good activity against P. aeruginosa (76.1% and 68.2%, respectively; but with inter-country variation). Ciprofloxacin resistance in E. coli and K. pneumoniae increased and needs close monitoring. Meropenem (98.2-99.8% susceptibility) and imipenem (88.8-99.4%) remained potent against important species of gram-negative bacteria from European ICUs actively using meropenem.