Due to a lack of longitudinal data the precise effect of long-term corticosteroid treatment on serum levels of the anabolic hormones Growth Hormone (GH) and Insulin-like Growth Factor-1 (IGF-1) is unknown. Therefore, we prospectively followed GH and IGF-1 levels over a six month period in 18 euthyroid patients with Graves' ophthalmopathy. Ten patients were treated with a three month course of prednisone; eight patients receiving retrobulbar irradiation served as controls. Baseline serum GH and IGF-1 levels were similar in both groups. Whereas no changes were seen in controls, prednisone induced a rapid increase in serum IGF-1 levels, which was sustained during the entire treatment period: 18.2 +/- 5.2 at baseline vs 24.1 +/- 6.7 nmol/l after 12 weeks of treatment (p < 0.001). After discontinuation of prednisone IGF-1 returned to baseline levels. Basal GH levels were frequently undetectable, but during treatment GH values were lower in the prednisone group (median of detectable values 4.0 mU/L [range 2.0-26.0]) than in controls (11.8 [2.8-44.0]; p < 0.01). In conclusion, long-term prednisone treatment is associated with suppressed GH levels and with an increase in total IGF-1 levels. This suggests that prednisone can disrupt the GH/IGF 1 axis.