Third generation cephalosporin antibiotics (3GC) have become the antibiotics of choice in many hospitals in recent years for the treatment of infections such as community-acquired pneumonia. However, increased use of 3GCs has also been associated with a rise in the occurrence of antibiotic-associated diarrhoea due to Clostridium difficile, as well as an increase in the prevalence of antibiotic resistant organisms such as methicillin resistant Staphylococcus aureus, vancomycin resistant entrococci, and extended-spectrum beta-lactamase-producing gram negative bacilli. In Western Australia, greater use of 3GCs was shown to correlate with more Clostridium difficile-associated diarrhoea (CDAD) in a large acute care teaching hospital during the 1980s. During the 1990s, the use of 3GCs in this hospital remained high and, at the end of 1998, a policy was introduced to prevent the use of ceftriaxone (the only 3GC in use) without prior approval. This resulted in a decline in 3GC use and a 50 per cent reduction in the incidence of CDAD during 1999 and 2000. To strengthen these observations, the impact of the 3GC policy on the occurrence of CDAD was analysed using time-series intervention analysis that showed a statistically significant decrease in the occurrence of CDAD during the post-intervention period after controlling for exogenous factors. Thus, changes in antibiotic prescribing practices can influence the incidence of CDAD and, potentially, antibiotic resistant pathogens.