OBJECTIVE To evaluate the in vitro caries inhibition effects of 1.23% APF foam topical fluoride treatment of cavity preparations, prior to restoration placement and after restoration placement. METHODS Sixty standardized Class V preparations were placed in molars. Randomly, 40 teeth received an amalgam restoration: Twenty other teeth had 1cc 1.23% APF foam applied to preparation surfaces for 1 minute, then an amalgam restoration was placed. The APF was not rinsed away prior to restoration, it was displaced by the pressure of the amalgam being condensed into the preparation. Twenty of the initial 40 amalgam restorations had 1 cc 1.23% APF foam applied to the external tooth/restoration margins for 4 minutes, the remaining 20 amalgam restorations acted as the controls. Acid-resistant varnish was placed, leaving 1 mm of tooth adjacent to restoration margins exposed. All specimens were subjected to an artificial caries challenge (pH 4.4) for 5 days. Sections of 100 microns were cut longitudinally through the restored margins, photographed under polarized light microscopy, then demineralized areas adjacent to restoration margins were quantitated. RESULTS Results demonstrated the mean (+/- S.D.) area (microns 2) of demineralization 100 microns from the restoration margins to be: amalgam 10,221 +/- 524, amalgam with 1-minute APF over preparation 529 +/- 557, amalgam with 4-minute APF over external surface 736 +/- 359. ANOVA and Duncan's (P < 0.05) indicated both 1.23% APF foam topical fluoride treatment regimens exhibited significantly less demineralization at restoration margins than the non-treated amalgam control. There was no significant difference between the two fluoride placement regimens examined in this study.