Antibiotic use data were analysed for surgical and medical intensive care units of seven university hospitals during the period 1998-2000. Applying the WHO/ATC definitions of defined daily doses overestimated the antibiotic use density when compared with prescribed daily doses. Mean antibiotic use density values (3-year averages) were 100 prescribed daily doses/100 occupied bed days in surgical intensive care, and 122 prescribed daily doses/100 occupied bed days in medical intensive care, respectively. Broadspectrum betalactams (like carbapenems), fluoroquinolones, glycopeptides and makrolides were more often prescribed in medical intensive care than in surgical intensive care. There were substantial differences in overall and specific drug class use between participating sites.