The effect of fluoride, released from, a fluoride-containing composite resin, on secondary caries was investigated by quantitative microradiography In all teeth investigated. less demineralization was found near the composite resin than was found at a distance of a few millimeters from the material after an acid attack. The presence of the fluoridereleasing composite resin reduced the lesion depth measured after an acid attack by about 35%. Therefore, in vivo, a reduction of recurrent caries may be expected around fluoride-containing composite resins.