To evaluate the relationship between obesity, diet, physical activity and breast cancer in Thai women, weconducted a case control study with 1,130 cases and 1,142 controls. Informed consent was obtained from allparticipants and a structured questionnaire was performed by trained interviewers to collect information ondemographic and anthropometric data, reproductive and medical history, residential history, physical activityand occupation as well as dietary habits. A significant positive association with an increased risk of breast cancerwas observed in women body mass index (BMI) of ≥25 mg/m2 (OR=1.33, 95%CI 1.07-1.65), the risk beinghigher in postmenopausal women (OR=1.67, 95%CI 1.24-2.25). In addition, underweight BMI at ages 10 and 20years showed an inverse association in all women (OR=0.70, 95%CI 0.56-0.88 and OR=0.74, 95%CI 0.59-0.93,respectively) and in those with a premenopausal status (OR=0.69, 95%CI 0.51-0.93 and OR=0.76, 95%CI 0.56-0.99, respectively). Regular exercise was associated with a decreased risk of breast cancer (OR=0.78, 95%CI0.68-0.98). Interestingly, analysis by type of activity revealed significant protective effects for women who reportedthe highest levels of walking for shopping (OR=0.58, 95%CI 0.38-0.88). High consumption of vegetables andfruit were associated with a decreased risk of breast cancer, while high consumption of animal fat showed anincreased risk in postmenopausal women. In conclusion, our results indicate that obesity and high consumptionof animal fat are associated with breast cancer risk, particularly in postmenopausal women, while recreationalphysical activity has protective effects. It seems that primary prevention of breast cancer should be promotedin an integrated manner. Effective strategies need to be identified to engage women in healthy lifestyles.