Two groups of children initially in the first and second grades chewed, rinsed with, and swallowed an acidulated phosphate-fluoride tablet containing 1 mg of fluoride either once or twice a day in school. A control group of children followed the procedure once a day using a placebo tablet. An interim evaluation after 55 months showed that the children receiving two fluoride tablets daily had a significant reduction in incremental caries on early erupting teeth, which received primarily topical exposure to the fluoride, as well as on late erupting teeth, which received mainly preeruptive, systemic exposure. The findings also suggested that the use of a single fluoride table daily may reduce dental caries, although the presence of a benefit was less definitive than it was for the two-tablet procedure.