OBJECTIVE To evaluate the effects of experimental xylitol dentifrices with and without fluoride on in vitro root caries formation. METHODS Root surfaces from caries-free human permanent teeth (n = 10) underwent debridement and a fluoride-free prophylaxis. The tooth roots were sectioned into quarters, and acid-resistant varnish was placed with two sound root surface windows exposed on each tooth quarter. Each quarter from a single tooth was assigned to a treatment group: (1) No treatment control; (2) Aquafresh Advanced (0.15% F = 1,150 ppm F); (3) Experimental xylitol dentifrice without fluoride (0.45% xylitol); and (4) Diamynt fluoride dentifrice with xylitol (0.83% sodium monofluorophosphate = 1,100 ppm F and 0.20% xylitol). Tooth root quarters were treated with fresh dentifrice twice daily (3 minutes) followed by fresh synthetic saliva rinsing over a 7-day period. Controls were exposed twice daily to fresh synthetic saliva rinsing daily over a 7-day period. In vitro root caries were created using an acidified gel (pH 4.25, 21 days). Longitudinal sections (three sections/tooth quarter, 60/group) were evaluated for mean lesion depths (water inhibition, polarized light, ANOVA, DMR). RESULTS Mean lesion depths were 359 +/- 37 microm for the control Group; 280 +/- 28 microm for Aquafresh Advanced; 342 +/- 41 microm for the experimental xylitol dentifrice without fluoride; and 261 +/- 34 microm for Diamynt. Aquafresh Advanced and Diamynt had mean lesion depths significantly less than those for the no treatment control and the experimental xylitol without fluoride dentifrice (P< 0.05). There were minimal non-significant differences in mean lesion depths between Aquafresh Advanced and Diamynt (P > 0.05).