BACKGROUND: We aimed to assess whether 2D:4D measures are associated with breast 
cancer risk.
METHODS: We derived the ratio of the lengths of the index and ring fingers 
(2D:4D), and right minus left 2D:4D (Δ(r-l)) from digit lengths measured from 
photocopies of participants' hands collected during a recent follow-up of the 
Melbourne Collaborative Cohort Study, a prospective study including 24 469 
women. Of the 9044 women with available data, we identified 573 incident breast 
cancer cases. Hazard ratios (HR) and 95% confidence intervals (CI) for a one 
standard deviation difference in 2D:4D measures were obtained from Weibull 
survival models, and linear regression models were used to examine potential 
associations between 2D:4D measures and age at menarche and menopause.
RESULTS: We found a direct association between left 2D:4D and breast cancer 
risk, an inverse association between Δ(r-l) and risk of breast cancer, but no 
association between right 2D:4D and breast cancer risk. Among breast cancer 
cases, both right 2D:4D and Δ(r-l) were inversely associated with age at 
diagnosis. We also observed associations between both right 2D:4D and Δ(r-l) and 
age at menopause, with increasing digit ratio measures related to earlier mean 
age at menopause.
CONCLUSION: Digit ratio measures might be associated with breast cancer risk and 
age at onset of breast cancer. If confirmed in other studies, this suggests that 
lower exposure or sensitivity to prenatal testosterone might be associated with 
lower risk of breast cancer.