The Meropenem Yearly Susceptibility Test Information Collection (MYSTIC) Program is a longitudinal global antimicrobial surveillance study that compares the activity of meropenem and comparator antimicrobial agents against pathogens isolated from intensive care, neutropenic or cystic fibrosis patients, and general wards. Data from the different European MYSTIC Program units (1997-2000) showed that the most prevalent isolates tested overall were methicillin-susceptible Staphylococcus aureus (MSSA; in accordance with study design methicillin-resistant S. aureus was not tested), Pseudomonas aeruginosa and Escherichia coli. In all the unit types, E. coli (approximately 20% having an extended spectrum beta-lactamase phenotype) and MSSA were highly susceptible to meropenem (97-99% susceptibility). Isolates of MSSA showed lower levels of susceptibility to ciprofloxacin (61-77% susceptibility) in both cystic fibrosis and neutropenia patients, and particularly high levels of resistance to ceftazidime (38% susceptibility) in cystic fibrosis units. Ciprofloxacin (54% susceptibility) and gentamicin (46% susceptibility) demonstrated low levels of activity against P. aeruginosa (frequently encountered in cystic fibrosis units). Meropenem and piperacillin/tazobactam were the most active agents against P. aeruginosa in all the unit types. Carbapenems and piperacillin/tazobactam have sustained > 90% susceptibility rates overall against the most frequently isolated pathogens. The analysis of specific units that house patients with a high-risk of contracting antimicrobial-resistant pathogens remains very important for the optimal selection of empiric regimens.