The lactic acid production in suspensions of plaque collected adjacent to orthodontic brackets retained with a glass ionomer cement (GIC), or a resin-based composite was investigated using a split-mouth technique. Forty-eight-hour-old plaque was collected at 3, 8, and 28 days, and 3 months after the onset of orthodontic treatment. Acid fermentation was induced by glucose and the L(+)-lactic acid concentrations were determined enzymatically after a 30-minute incubation period. Significantly (P < 0.05) lower levels of lactic acid were found in plaque from GIC-retained brackets compared with the composite controls at the 28 days and 3 months sampling occasions, respectively. The results suggest that the use of GIC as a bonding agent in orthodontics can be beneficial for patients assessed at risk of caries development.