Insulin-like growth factor-1 (IGF-1) is capable of stimulating breast cancer cell growth in vitro and the presence of IGF-1 receptors has been demonstrated in primary breast cancers. We determined plasma IGF-1 in a primary breast cancer population and in a control population. Radioimmunoassays were performed either directly on plasma, IGF-1 (NE), or after an acid-ethanol extraction of the plasma, IGF-1 (E). We demonstrated an inverse correlation between age and IGF-1: for this reason, only results obtained in females of the same age range (> 35 years) were compared. Median concentrations of IGF-1 were significantly higher in primary breast cancers [IGF-1 (E) = 152 ng/ml, IGF-1 (NE) = 26 ng/ml, n = 44] than in controls [IGF-1 (E) = 115 ng/ml, IGF-1 (NE) = 20 ng/ml, n = 92]. To our knowledge such a growth factor increase has never been described in breast cancer. We conclude that IGF-1 could be an important factor involved in the development of breast cancer and that treatment reducing IGF-1 levels could be beneficial for patients.