Fluoride concentrations were studied in enamel biopsies from maxillary central incisors 6 weeks and 18 months after fluoride treatment. In the short-term study biopsies were obtained prior to and after treatment with acidic sodium or ammonium fluoride. The findings showed that large amounts of fluoride were deposited in enamel from NH4F treatment (mean concentration 84,723 ppm), indicating substantial CaF2 formation. NaF treatment resulted in mean fluoride concentrations of 7,818 ppm. In the 18-month study, biopsies from 58 placebo-treated teeth were analyzed for total fluoride (mean 1,733 ppm). Twenty-five additional biopsies from placebo-treated and 58 from NH4F-treated teeth were analyzed for KOH-soluble (CaF2) and KOH-nonsoluble (apatitic) fluoride. The mean values for total fluoride were 1,669 and 2,085 ppm in the placebo-treated and in the NH4F-treated groups, respectively. The corresponding mean values for KOH-nonsoluble fluoride were 1,467 and 1,731 ppm and for KOH-soluble fluoride 202 and 354 ppm, respectively. The increase in enamel fluoride after fluoride treatment was only marginally significant. Biopsies from the ammonium fluoride treated group were significantly more likely to have high (30 vs. 8%) and moderate (28 vs. 16%) CaF2 levels and less likely to have low levels than biopsies of placebo-treated teeth (chi-square = 8.0 with 2 d.f; p = 0.018). It is concluded that very substantial amounts of CaF2 are present in enamel 6 weeks after treatment, and small amounts may persist in the surface enamel for as long as 18 months.