The use of antibiotics in Dutch hospitals between 1991 and 1996 was investigated. A total of 54 hospitals responded to the enquiry, representing over 70% of all hospital beds in The Netherlands. The use of antibiotics in Dutch hospitals, expressed as defined daily doses (DDD) per hundred bed days, gradually increased from 37.2 DDD per 100 bed days in 1991 to 42.5 DDD per 100 bed days in 1996. The antibiotic that showed the largest increase in use was co-amoxiclav. Its use increased more than three-fold from 3.93 DDD per 100 bed days in 1991 to 12.5 DDD per 100 bed days in 1996. The increase in use of co-amoxiclav exceeded the increase in total antibiotic consumption. The use of cephalosporins remained fairly constant during the study period, but there were changes in the relative use of the different cephalosporin groups. The use of earlier cephalosporins gradually decreased, whereas the use of the more recently developed cephalosporins increased between 1991 and 1996. Ciprofloxacin and norfloxacin were the most commonly used fluoroquinolones throughout the study period. The use of ofloxacin increased significantly between 1991 and 1996, approaching the levels of use of ciprofloxacin and norfloxacin. There may be complex reasons for the increases, which need further analysis, but they mirror those few data available from elsewhere in the world. Possible explanations include more intensive treatment to expedite patient discharges, sicker patients with more serious infections and more resistant organisms.