We present three cases of malignant solitary fibrous tumors of the pleura (SFTP) that produced recurrent hypoglycemia. Removal of the tumors produced normoglycemia. The tumors were well circumscribed and lobulated, and consisted of firm masses weighing 1,150 g to 1,450 g with the greatest diameter of 15 to 20 cm. The tumors were composed of spindle cells in fascicles or in a haphazard arrangement and were highly cellular and mitotically active (3-8 mitoses/10 high-power fields), showing histologically malignant features. Ultrastructurally, fibroblastic features of the tumor cells were present. Insulin-like growth factors (IGF) have been implicated in the presentation of hypoglycemia. The serum insulin and C-peptide levels were not elevated. Serum IGF-I levels were also low with values of 97.4, 157.1 and 51.9 ng/mL (ref. 125-317 ng/mL), respectively. However, tumor cells were strongly positive for IGF-I receptor on immunohistochemical analysis. It is tempting to speculate that IGF-I contributes to the hypoglycemia, even though the circulating levels were low.