We analyse an empirical case study of an inter-organizational information system (IOIS) in the Australian pharmaceutical distribution industry, using a theoretical data coding approach, to provide a concise grounded account of changes in the material, normative and ideational structures within the participating practices over a 25 year period as the IOIS evolved from a proprietary closed system to a quasi-open shared ordering platform. We find evidence that the resilience of the IOIS over this long time period is explained by a layered accumulation of new structures at the level of individual practices, while the punctuated evolutionary change accompanied the appearance of a new practice, historically connected to the incumbent practices. These findings are in substantial agreement with systems evolution mechanisms proposed by Porra (1999). Understanding IOIS evolution will be important for the provision of key enabling information infrastructures envisioned in existing and planned ICT-mediated healthcare initiatives.