BACKGROUND: There has been a rapid increase in proton pump inhibitor (PPI) prescribing in recent years, and controlling the cost and improving the quality of prescribing is an issue of concern to many GPS: OBJECTIVE: Our aim was to compare GPs' usage of different PPIs and explore how GPs' PPI prescribing changes following the introduction of a cheaper competitor. METHODS: PPI prescribing data (PACT) for 53 GPs, who were selected as regular users of a teaching hospital, were monitored from January 1995 to December 1997. The GPs were located in two adjoining health districts and had been interviewed about influences on their decisions to begin prescribing lansoprazole. The PPI prescribing data were collected for the teaching hospital and the general hospital in the adjoining district. RESULTS: Complete prescribing data were available for 50 GPS: Total PPI prescribing increased throughout the study due mainly to increasing use of the new PPIS: Use of the new PPIs increased from 6 to 24% over 3 years. The proportion of maintenance doses prescribed increased from 3 to 12%. There was a 23-fold difference in total PPI prescribing and an 87-fold difference in lansoprazole prescribing between the highest and lowest prescribers. The uptake of pantoprazole was slower than that of lansoprazole. A rapid increase in the use of lansoprazole by the GPs followed an increase in use in the teaching hospital. CONCLUSION: Hospital prescribing was an important influence on the choice of PPI used by GPS: The wide variation in PPI prescribing suggests that there is scope for improvement in the quality and cost of PPI prescribing.