Breast cancer is common malignancy in Thai female. Although there are well established risk factors, manyenvironmental agents with an impact are still unknown especially with reference to occupation. The objective ofthis 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 ofcases and controls were recruited at the Thai National Cancer Institute, Khon Kaen University Hospital andKhon Kaen Provincial Hospital during 2002-2004. Cases were newly diagnosed with histological confirmed breastcancers while controls were selected from healthy women matched by age (±5 years) and geographical area. Afterinformed consent was signed, information was obtained on occupation and other risk factors from each subjectusing an interviewer-administered and structured questionnaire. The International Standard Classificationof Occupations version 1968 (ISCO-68) was used to code for occupational categories. The relation betweenoccupational categories and breast cancer risk was evaluated by unconditional logistic regression analysis. Themean age of cases and controls were 46.9±10.6 and 47.8±9.9 years, respectively. Fifty-five percent of cases werepre-menopausal women. After adjusting for confounding factors by multiple logistic regression analysis, theresults showed that occupational category as production and related workers, transport equipment operatorsand labourers was associated with an increased risk of breast cancer (OR=1.41 95% CI=1.01-1.97) and thisfinding was also supported by a statistically significant positive trend for duration of employment (p=0.01). Asignificantly 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 todevelop breast cancer. Further studies are needed to assess occupational exposure in specific occupations.