The . Meropenem Yearly Susceptibility Test Information Collection (MYSTIC) Programme in the fifth year continues to monitor the spectrum of activity and potency of meropenem within medical centers where carbapenems are used for the treatment of serious infections. The antimicrobial activity of 11 broad-spectrum agents (including initial comparisons for levofloxacin) was assessed against 2,848 isolates in 2003. The minimum inhibitory concentration (MIC) results demonstrate the continued high potency of meropenem against all monitored pathogens. Against all Gram-negative bacilli tested, the overall rank order of susceptibility was meropenem (%) > imipenem (%) > cefepime (%) > tobramycin (%) > piperacillin/tazobactam (%) > ceftazidime (%) > gentamicin (%) > levofloxacin (%) > ciprofloxacin (%) > aztreonam (%) > ceftriaxone (%). Clonal-based resistances were observed that adversely influenced carbapenem resistance rates, particularly among Klebsiella spp. and Acinetobacter baumannii isolates. Continued surveillance of the carbapenem class and other broad-spectrum agents is warranted to monitor activity against pathogens causing serious infections in hospitalized patients.