Breast cancer incidence may be increasing in Thailand but very little research has assessed core breast cancer risk factors in this country. We used baseline questionnaire data from a national cohort study of Thai Open University students in an exploratory case-control study of breast cancer. The study included 43 female cases and 860 age-matched controls selected from the remaining 47,271 female cohort participants. Odds ratios and 95% confidence intervals were calculated using conditional logistic regression. The women were predominantly premenopausal. Taller women had an increased risk of breast cancer (OR=2.3, 95% CI 1.1-4.8, for height >or=160cm vs <or=154cm) as did women with non-insulin dependent diabetes mellitus (OR=8.4, 95% CI 1.7-41). Women with older siblings had a reduced risk of breast cancer compared to those firstborn (OR=0.3, 95% CI 0.2-0.7). Although limited by small case numbers, our findings suggest substantial increases in breast cancer rates in Thailand could be expected in the future.