To evaluate the relationship between obesity, diet, physical activity and breast cancer in Thai women, we conducted a case control study with 1,130 cases and 1,142 controls. Informed consent was obtained from all participants and a structured questionnaire was performed by trained interviewers to collect information on demographic and anthropometric data, reproductive and medical history, residential history, physical activity and occupation as well as dietary habits. A significant positive association with an increased risk of breast cancer was observed in women body mass index (BMI) of ≥ 25 mg/m2 (OR=1.33, 95%CI 1.07-1.65), the risk being higher in postmenopausal women (OR=1.67, 95%CI 1.24-2.25). In addition, underweight BMI at ages 10 and 20 years 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%CI 0.68-0.98). Interestingly, analysis by type of activity revealed significant protective effects for women who reported the highest levels of walking for shopping (OR=0.58, 95%CI 0.38-0.88). High consumption of vegetables and fruit were associated with a decreased risk of breast cancer, while high consumption of animal fat showed an increased risk in postmenopausal women. In conclusion, our results indicate that obesity and high consumption of animal fat are associated with breast cancer risk, particularly in postmenopausal women, while recreational physical activity has protective effects. It seems that primary prevention of breast cancer should be promoted in an integrated manner. Effective strategies need to be identified to engage women in healthy lifestyles.