This study investigated the effects of acidic soft drinks on the resistance of metal brackets to shear forces in vitro and in vivo. Thirty noncarious maxillary premolar teeth, scheduled for extraction for orthodontic purposes, were used in the in vivo group. Thirty other noncarious maxillary prernolar teeth, already extracted for orthodontic purposes, were used in the in vitro group. The teeth in both groups were divided equally in three subgroups, ie, the Coca-Cola (R), Sprite (R), and control subgroups. Brackets were bonded using conventional methods. Teeth in the in vivo group were rinsed with the acidic drink three times for five minutes daily and extracted after three months. Teeth in the in vitro group were kept in the acidic drink for five minutes on three equal time intervals within 24 hours. The brackets from both groups were subjected to shearing forces using a Universal test machine. After the shearing tests, a scanning electron microscope was used to determine the amount and the localization of erosion. The results indicated that both acidic soft drink subgroups had a reduced debonding resistance in vivo and in vitro compared with their control subgroups. No statistical difference in debonding resistance was found between the in vivo and in vitro groups. Areas of defect due to erosion were observed on the enamel surface around the brackets in both the in vitro and in vivo groups. Acidic soft drinks such as Coca-Cola (R) and Sprite (R) have a negative effect on bracket retention against shearing forces and enarnel erosion.