Fluoride uptake, topographic distribution, and effect on human dental enamel following topical treatment (for 4 min, 8 min and 72 h) with an acidulated fluoride-phosphate solution have been studied by means of electron probe X-ray microanalysis and X-ray diffraction. The samples examined were divided into a non-washed group, and a washed group from which excessive and loosely bound F had been eliminated from the enamel. The topographic distribution of fluorine was demonstrated by electron probe X-ray microanalysis. In the non-washed samples, an outer layer with a higher concentration of F, and adjacent to this another layer having a moderate concentration of F, were observed. After a prolonged washing about 90% of F of the outer layer was eliminated, whereas that of the inner layer persisted. X-ray diffraction analysis shows the presence of CaF2 in non-washed samples; the lines of CaF2 disappear after prolonged washing. On the other hand, a displacement of the apatite lines was observed in all treated samples, corresponding to a shortening of the parameter a of the unit cell, due to the replacement of the OH–– by F–– in the crystalline network. These results show that by topical treatment of enamel with an acidulated fluoride-phosphate solution, an accumulation of F in the surface layers of the enamel is produced, partly as CaF2 and partly as fluor