Analyses of breast-cancer risk in relation to the molar ratio of IGF1 to IGFBP3 showed a significant positive association, but the magnitude was less than for the analyses of IGF1; ORs in increasing fifths of the ratio were 1·17 (95% CI 1·04–1·32), 1·12 (0·99–1·26), 1·14 (1·01–1·29) and 1·23 (1·08–1·40) (test for trend p=0·009). The gene discussed is IGF1; the disease is breast cancer.