The 2-year caries increments of fifth to seventh grade students (mean age 11.7 years) residing in two fluoride-deficient communities (F < 0.1 ppm) on Long Island, N.Y., were compared relative to their use of different fluoride dentifrices. Group I used a conventional Na2PO3F dentifrice (1,000 ppm F) and served as the positive control. Group II used a mixed fluoride dentifrice containing equimolar amounts of NaF and Na2PO3F which provided a total fluoride concentration of 1,000 ppm. Group III also used a mixed fluoride dentifrice containing equimolar amounts of NaF and Na2PO3F with a total fluoride concentration of 2,500 ppm. Brushing was performed unsupervised at home for 23 months. There were no statistically significant differences (F test) in the 2-year caries increments between the dentifrice groups for DMFT, DMFS, and DMFS of specific tooth surface types. Mitigating factors which may have influenced the results were the low proximal caries increments and the degree of compliance. Use of a mixed fluoride dentifrice (NaF/Na2PO3F) at a standard 1,000 ppm F concentration or at 2.5 times standard did not provide superior caries inhibition compared to a conventional Na2PO3F dentifrice containing 1,000 ppm F.