PURPOSE: The aim of this study was to determine the mechanisms of failure of magnets used for denture retention. MATERIALS AND METHODS: Magnets were retrieved from dentures that had failed after 4 to 18 months of clinical use. The magnets were sectioned and the failure modes were evaluated using optical or scanning electron microscopy. RESULTS: The magnetic retention system investigated consists of a neodymium-iron-boron magnet sandwiched in a stainless steel keeper, capped with a titanium dome, and sealed with epoxy resin. The diffusion of moisture through the epoxy seal and the breakdown of the encapsulating material led to corrosion of the magnet. In both cases, a loss in retention occurred as the corrosion products weakened the overall magnetic attraction. CONCLUSION: The failure of magnets may occur by either breakdown of the epoxy seal or breakdown of the encapsulating material. Both problems need to be addressed to prolong the life span of these magnets.