Breast cancer is common malignancy in Thai female. Although there are well established risk factors, many environmental agents with an impact are still unknown especially with reference to occupation. The objective of this study was to investigate the risk of female breast cancer among different occupational categories in Thailand. A frequency-matched case-control study was conducted among Thai women aged 17-79. A total of 516 pairs of cases and controls were recruited at the Thai National Cancer Institute, Khon Kaen University Hospital and Khon Kaen Provincial Hospital during 2002-2004. Cases were newly diagnosed with histological confirmed breast cancers while controls were selected from healthy women matched by age (± 5 years) and geographical area. After informed consent was signed, information was obtained on occupation and other risk factors from each subject using an interviewer-administered and structured questionnaire. The International Standard Classification of Occupations version 1968 (ISCO-68) was used to code for occupational categories. The relation between occupational categories and breast cancer risk was evaluated by unconditional logistic regression analysis. The mean age of cases and controls were 46.9 ± 10.6 and 47.8 ± 9.9 years, respectively. Fifty-five percent of cases were pre-menopausal women. After adjusting for confounding factors by multiple logistic regression analysis, the results showed that occupational category as production and related workers, transport equipment operators and labourers was associated with an increased risk of breast cancer (OR=1.41 95% CI=1.01-1.97) and this finding was also supported by a statistically significant positive trend for duration of employment (p=0.01). A significantly decreased breast cancer risk was observed in clerks (OR=0.59, 95% CI=0.37-0.96). In conclusion, this study revealed that women who have lifetime occupation in an industrial setting may have higher risk to develop breast cancer. Further studies are needed to assess occupational exposure in specific occupations.