This study aimed to compare caries-like lesions produced aroung fluoride-containing amalgam and conventional amalgam fillings in vitro. Class V cavities were prepared in 90 extracted premolars, a fluoride-containing amalgam was inserted in 45 cavities (experimental group) and a conventional amalgam in 45 cavities (control group). After three months in an acidified gelatin solution, a longitudinal ground section passing through the cavity was prepared from each tooth. The sections were imbibed in water, quinoline and some in air before examination by polarized light microscopy. When imbibed in Canada balsam the sections were photographed and areas of demineralization in the enamel measured morphometrically. Some sections were studied by microradiography and electron microprobe analysis. Numerical scoring of the histological observations revealed significantly lower values for enamel and dentin cavity wall lesions in the experimental group, while the scores for the outer lesions were not statistically different between the two groups. The mean extension of enamel wall lesions was significantly lower in the experimental group, while no difference was observed in the outer lesions. A zone of increased mineralization in the enamel cavity walls was a frequent finding in the experimental group. This study has shown that fluoride-containing amalgam has an inhibitory effect on the development of experimental cavity wall lesions in vitro.