A prospective controlled clinical trial was undertaken to evaluate the effectiveness of stannous fluoride-releasing elastomeric modules (Fluor-I-Ties) and chain (Fluor-I-Chain) in the prevention of enamel decalcification during fixed appliance therapy. Forty-nine patients (782 teeth) were included in the experimental group, where the fluoride-releasing elastomerics were used. Forty-five patients (740 teeth) who received non fluoride-releasing elastomerics formed the control group. All patients had their elastomerics replaced at each visit. Enamel decalcification incidence and distribution were recorded using an index by direct clinical observation. In the control group enamel decalcification occurred in 73 per cent of patients and in 26 per cent of all teeth. In the experimental group the corresponding incidence was 63 and 16 per cent, respectively. The overall reduction in score per tooth produced by the fluoride-releasing elastomerics was 49 per cent, a highly significant difference (P < 0.001). A significant difference was seen in all but the occlusal enamel zones. The majority (over 50 per cent) of lesions occurred gingivally. The teeth most severely affected were the maxillary lateral incisors and mandibular second premolars. There was no difference in treatment duration between groups.