The effects of treatment with the aromatase inhibitors aminoglutethimide (AG) and formestane or the synthetic progestin megestrol acetate (MA) on plasma levels of insulin-like growth factor I (IGF-1), IGF-II, IGF-binding proteins (IGFBPs), and IGFBP-3 protease status were investigated in 39 patients suffering from advanced breast cancer. Treatment with AG and MA elevated plasma levels of IGF-I by mean values of 27% (n = 15; P < 0.025) and 81% (n = 7; P < 0.025), respectively, whereas treatment with formestane had no effect (n = 13). Treatment with AG increased plasma levels of IGFBP-2, as evaluated by Western blotting (P < 0.01). MA caused a significant reduction in IGFBP-3 protease activity (mean reduction, 69%; P < 0.05). These alterations in plasma IGF-I and IGFBP-3 protease activity were reversed 4 weeks after terminating MA therapy (n = 8; P < 0.025). Taken together, 13 of 15 patients had reduced IGFBP-3 protease activity during treatment with MA compared to the control situation (P < 0.0025). Total levels of IGFBP-3 as measured by RIA were moderately elevated by treatment with MA (mean increase, 19%; P < 0.05), and Western immunoblotting revealed an increase in the amount of intact IGFBP-3 and reduced amounts of IGFBP-3 in the modified form. None of the treatment modalities had any influence on plasma levels of IGF-II. The increase in the plasma IGF-I concentration seen during treatment with MA may be secondary to an increased level of intact IGFBP-3. This could reflect an alteration in IGF availability that contributes to the antitumor effect of MA.