Background: Traditional Chinese Medicine Constitution (TCMC) refers to an integrated, metastable and natural specialty of individual in morphosis, physiological functions and psychological conditions.It has been suggested that unbalanced TCMCs largely contribute to individual susceptibility to various diseases.This study aimed to investigate the distribution of the TCMCs of healthy Chinese women and breast cancer women in Hong Kong, and thus to evaluate the efficacy of TCMC on the occurrence of breast cancer.Methods: A retrospective, cross-sectional and case-control matched design was utilized in this study.Chinese women with breast cancer and healthy Chinese women were recruited and then assessed using the Traditional Chinese Medicine Physical Constitution Scale for their TCMC types.Social-demographic information and related risk factors for breast cancer were collected through structured questionnaires.Multiple factor analysis was used to explore the association of unbalanced TCMC types with breast cancer.Results: 305 healthy women and 305 breast cancer patients were involved in this study.Compared with healthy women, the proportion of four unbalanced constitutions including Qi-depressed, Qi-deficiency, Wetness-heat and Yin-deficiency constitutions in breast cancer women was significantly higher than those in healthy women (p values less than 0.05).However, the results of stepwise logistic regression analysis indicated that only Qi-depressed constitution was significantly positively correlated with breast cancer (OR=3.21, CI 1.49-6.92), the correlation increased when Qi-depressed constitution combined with Wetness-heat constitution (OR=4.82, CI 1.83-12.71) or Blood-stasisconstitution (OR=3.27, CI 1.31-8.16).Moreover, the complex of Wetness-heat constitution and Blood-stasis constitution may also closely related to breast cancer (OR=2.88, CI 1.02-8.17)Conclusion: Qi-depressed constitution can act as an independent risk factor for breast cancer.Wetness-heat constitution and Blood-stasis constitution may also play the role in the occurrence of breast cancer when combined with Qi-depressed constitution or with each other.This indicates that the unbalanced TCMC types could explain the variability in individual susceptibilities to breast cancer, suggesting the feasibility to develop a novel approach from Chinese medicine in the prevention of breast cancer.