Much of the research into composite adhesives suggests that these materials will deteriorate in the oral environment, but most of these claims are made on the basis of extrapolation from in vitro experiments; relatively little in vivo research has been conducted into the mechanical properties of composite adhesives. For this study, we recruited 20 volunteers to wear removable appliances containing orthodontic brackets bonded to enamel slabs for 12 weeks. Each appliance carried 3 brackets bonded with Transbond (3M Unitek UK; Bradford, United Kingdom) and 3 brackets bonded with Heliosit (Ivoclar-Vivadent; Leicester, United Kingdom). The bond strengths were tested at intervals of 4, 8, and 12 weeks. Control specimens were stored in sterile water at 37 degrees C and were debonded at the same time intervals. Transbond specimens debonded after 4 weeks in vivo had significantly (P <.05) lower bond strengths (9.78 megapascals [MPa]) than did the controls (14.34 MPa). In vivo, specimens bonded with Heliosit had significantly (P <.05) lower bond strengths after 4 weeks (8.16 MPa vs 10.96 MPa) and 8 weeks (9.96 MPa vs 13.61 MPa) than did the controls. These results indicate differences between bond strength testing in vitro and in vivo. Further research is required into the effects of the oral environment on bond strength.