As multiparity is protective against breast cancer in the developed world but not in the developing world, as the IGF1-19/-19 genotype is more common among black, Indian-Pakistani and other Asian women than among white women, and as the IGF1-19/-19 genotype inverts the effect of several non-genetic factors on breast cancer risk, we aimed to investigate whether the effect of multiparity on age at breast cancer diagnosis is influenced by the IGF1-19/-19 genotype. This evidence concerns the gene IGF1 and breast cancer.