The fluoride concentrations in dental plaque adjacent to orthodontic brackets retained with a glass ionomer cement (GIC) or a resin-based composite were investigated using the split-mouth technique. 48-hour plaque was collected at 3, 8, 28 days and 6 months after the onset of orthodontic treatment. The fluoride content of the plaque samples was determined after a microdiffusion procedure with a fluoride-sensitive electrode. Significantly (p < 0.001–0.05) elevated concentrations of fluoride were found in the plaque samples collected adjacent to GlC-retained brackets compared to plaque sampled from composite-retained brackets on all sampling occasions. The results suggest that GIC-bonded brackets in orthodontic treatment may act as local long-term fluoride-releasing devices.