Background:A positive association between socio-economic position and breast cancer has been widely observed, but not hitherto within twin pairs, where shared familial factors were adjusted for.Methods:We used data on education and other factors from the Danish Twin Registry, The Danish Cancer Registry, and official registers on a total of 16¿310 twins. Unpaired and intrapair Cox regression analyses were compared.Results:In the unpaired analysis, an educational gradient in breast cancer risk was found. Similar results were seen in the intrapair analyses of all twins, although no longer statistically significant. When intrapair analyses were stratified on zygosity, the effect of education was attenuated in the monozygotic twins.Conclusion:The main findings support an effect of education beyond shared familial factors.