Most studies on women with breast cancer indicate that obesity is positively associated with late-stage disease. Some results have shown a similar relationship between breast size and stage. A recent study found that the association between body mass index (BMI) and stage was limited to cancers that were self-detected, suggesting that the BMI-stage relation may be due to delayed symptom recognition. We examined the relationships between stage and both BMI and breast (bra cup) size, stratified by method of detection, using data from a population-based case-control study of 1,361 women (ages 20-44 years) diagnosed with breast cancer during 1990-1992. Height and weight measurements and information on bra cup size, method of cancer detection and other factors predictive of stage at diagnosis were collected during in-person interviews. A case-case comparison was conducted using logistic regression to estimate odds of regional or distant stage rather than local stage in relation to BMI and bra size. Odds of late-stage disease were increased with higher BMI [adjusted odds ratio (OR) for highest to lowest tertile = 1.46, 95% confidence interval (CI) 1.10-1.93] and larger bra cup size (OR for cup D vs. cup A = 1.61, 95% CI 1.04-2.48). These relationships were not modified by the method of detection. Differences in etiologic effects, rather than differences in detection methods, may explain the relations observed between stage and both BMI and breast size.