A sample of 165 extracted or exfoliated teeth containing amalgam restorations were categorized as to the degree of marginal integrity of the amalgam. Microstructures of the samples representing the highest (grade 11) and lowest (grades 1 and 2) degrees of marginal deterioration were studied by SEM/EDS. The clinical evaluation and microscopic evaluation indicated that samples showing poor marginal integrity contained large quantities of tin-rich and tin-chlorine-rich corrosion products with little or no gamma-2 phase remaining. Obvious signs of fracture and cracking at the cavosurface were always present. Products containing calcium were extensively associated with corrosion products containing tin, and these were found predominantly at the tooth-amalgam interface. No copper-rich amalgams were found in this grouping. Amalgams with good marginal integrity had no microscopic cracks or fractures and generally little evidence of corrosion was found. However, one of six samples had extensive corrosion products, indicating that corrosion product formation is not the only factor contributing to marginal deterioration. After 6 years of clinical use, the degree of marginal deterioration did not appear to depend unerringly on time of use. Further work is necessary to define the microstructural characteristics of samples showing intermediate amounts of marginal deterioration and to determine if copper-rich amalgams consistently have small to moderate amounts of marginal deterioration.