Objectives: The purposes of this study were to compare the fluoride ion release from a freshly mixed polyacid-modified resin composite, or 'compomer' (Dyract), and 3 resin-modified glass-ionomer cements (Fuji II LC, Photac-Fil, and Vitremer) and to compare the use of 3 units for measuring fluoride release. Method and materials: Five specimens (3.0 3 2.7 mm) of each material were prepared. The specimens were placed in polypropylene vials with 2 mL of deionized water and stored at 37°C. The solutions were replaced weekly and the levels of fluoride ions were analyzed at days 1, 7, and 30 and subsequently every 28 days for 253 days. Fluoride measurements were carried out using a fluoride ion-selective electrode connected to a pH ion-selective electrode meter. Fluoride ion release was measured in parts per million, micrograms per square centimeter, and micrograms per cubic millimeter. Results: Fuji II LC, Photac-Fil, and Vitremer showed high initial release values, which decreased exponentially and then showed a slow decline during the ensuing time. Dyract released significantly less fluoride ions during the first 84 days than did the 3 resin-modified glass-ionomer cements and maintained this low level of release throughout the study period. The amounts of fluoride ion release measured at any time interval varied with the units of measurement chosen, but the pattern of release remained the same. Conclusion: There was a wide variation in the amounts of fluoride ions released from related products, but the patterns of release were similar and unaffected by the units of measurement used.