Bioactive amorphous calcium phosphate (ACP) composites show promise as remineralizing dental materials. Objective: To assess the in vitro remineralizing efficacy of ACP composite for orthodontic applications. Methods: The lesions were artificially created in extracted human molars. Single tooth sections approx. 120 mm thick were cut and individually placed in holders exposing only the carious enamel surface. The exposed surfaces were coated with 1) the experimental ACP composite (40 mass% zirconia-hybridized ACP and 60 mass % photo-activated resin), 2) commercial fluoride-releasing orthodontic cement or 3) left untreated (control). The composite-coated sections were photo-cured, and microradiographic images were taken of all three groups of specimens before the treatment. Specimens were then cyclically immersed in demineralizing and remineralizing solutions for one month at 37oC to simulate the pH changes occurring in the oral environment. Microradiographs of all specimens were taken after the treatment. Data were statistically analyzed by one way ANOVA and all pair-wise multiple comparisons. Results: Quantitative digital image analysis of the matched areas from the contact microradiographs taken of the sections before and after treatment indicated higher mineral recovery (14.4 %) with ACP composites compared to the commercial orthodontic F-releasing cement (4.3 %) while the control specimens showed, on average 55.4 % further demineralization. Conclusions: The experimental ACP orthodontic composite efficiently established mineral ion transfer throughout the body of the lesions and restored the mineral lost due to acid attack. It can be considered a useful adjuvant for the control of caries in orthodontic applications. Support: NIDCR Grant DE13169-09 and ADAF.