Serum IGF-I concentration was not associated with breast cancer risk, either before or after adjustment for potential reproductive and other confounders; however, after statistical adjustment for serum IGFBP-3 concentration, women in the highest third of IGF-I concentration had an increased risk compared with women in the lowest third, although this was not statistically significant (OR 1.71; 95% CI: 0.74–3.95; test for linear trend; P=0.21). The gene discussed is IGFBP3; the disease is breast cancer.