The independent contributions of formula and water to the total fluoride (F) intake from the diet of formula-fed infants is not fully documented. Although the precise timing and mechanism by which dental fluorosis occurs has not been fully defined, water F levels can be an important consideration in the risk of dental fluorosis for formula-fed infants. An assessment of 1,308 participants younger than 2 years old revealed that: 81% of homes received public water; 19% received well water; 26% of participants used bottled water; and 11% used some kind of filtration system. In this study, virtually all formulas consumed by the birth cohort and water sources used in the reconstitution of these formulas were assayed for F using a F ion specific electrode and direct read method, except for soy-based formulas, which were analyzed by microdiffusion (modified Taves). Among 78 commercially available bottled waters in Iowa, F levels ranged from 0.02 to 1.36 ppm (mean 0.18 ppm), 83% from 0.02 to 0.16 ppm, 7% from 0.34 to 0.56 ppm, 1% had a F level of 0.88, and 9% had F levels > 1.0 ppm. Among 47 casein (milk)-based formulas, 16 ready-to-feed (RTF) formulas had levels of 0.04-0.55 ppm F (mean 0.17 ppm), 14 liquid concentrates (LC) reconstituted with distilled water had levels of 0.04-0.19 ppm F (mean 0.12 ppm), and 17 powdered concentrates (PC) reconstituted with distilled water had levels of 0.05-0.28 ppm F (mean 0.14 ppm). The 17 soy-based formulas had a range of 0.04-0.47 ppm F (mean 0.26 ppm).(ABSTRACT TRUNCATED AT 250 WORDS)