This study was designed to evaluate time and dose dependency of alterations in insulin-like growth factor (IGF)-I, free IGF-I and the functional status of IGF-binding protein (IGFBP)-3 in breast cancer patients during treatment with megestrol acetate (MA). In 16 patients receiving MA 160 mg daily, total IGF-I levels increased gradually (significant after 3 days on treatment) by a maximum of 2.66-fold after 5-6 months on treatment. However, free (readily dissociable) IGF-I levels increased to a smaller extent (1.23-2.15-fold). This discrepancy may be due to an increase in intact IGFBP-3 determined by Western ligand blotting (WLB). Similar findings were observed in 12 patients treated with MA in escalating doses from 40-800 mg daily. A dose-dependent increase in IGF-I was observed up to a dose level of 120 mg daily. We conclude that treatment with MA caused a profound increase in plasma levels of total IGF-I accompanied by a moderate increase in free IGF-I. This may explain the anabolic effects of MA in patients suffering from cachexia, but refute the hypothesis that alterations in the IGF-system may contribute to the antitumour effects of MA in breast cancer patients.