INTRODUCTION: To examine the long-term effects of normal saline and a synthetic tissue fluid (phosphate buffered saline, PBS) on mineral trioxide aggregate (MTA). MATERIALS AND METHODS: Root-ends of twelve extracted human teeth were resected; root-end cavities were prepared and filled with MTA. Samples were randomly divided into two groups of six each. Teeth in group I were placed in normal saline, whilst teeth in group II were placed in PBS. After five months, elemental analysis of the surface of the root-end filling was performed using electron probe microanalysis (EPMA). RESULTS: Results showed that all teeth kept in PBS formed crystal deposits. In contrast, no such crystal formation was observed in teeth kept in normal saline solution. The results of elemental analysis showed that the composition of the crystals observed for teeth kept in PBS was consistent with that of a mineral hydroxyapatite, Ca10(PO4)6(OH)2. CONCLUSION: Based on these in vitro results, we suggest that the hydration of MTA surface and the release of calcium from MTA in contact with phosphorous of PBS produced hydroxyapatite crystals over MTA and it may be a mechanism which is responsible for cementum formation during in vivo studies.