Bovine enamel slabs containing artificial caries-like enamel lesions were placed in partial dentures and worn by patients using either a 0 or a 1,500 ppmF (as MFP) toothpaste for a period of 6 weeks. At the end of the in vivo period, fluoride uptake, susceptibility to demineralization and mineral content-depth profiles were measured for the 2 experimental groups in comparison with the baseline lesions. Fluoride content values were 12.3, 5.3 and 3.3 μg/cm2 for the fluoride, the control and the baseline groups, respectively. Demineralization susceptibility values (expressed as calcium output in acid buffer in arbitrary units) were 1.16, 1.64 and 2.02 for the fluoride, the control and the baseline groups, respectively. Microradiographic analysis of the lesions revealed evidence of significant net de- or/ and remineralization in some subjects in both groups, when compared with the baseline lesions, indicating that either process may dominate in an individual. However, on average no difference in mineral content of the lesions was found for the three groups.