The authors report a case of pleural fibrosarcoma associated with severe hypoglycaemia. Chromatography of the patient's serum and tumour fragments revealed a substance with a high molecular weight and an insulin-like activity. Professor Poffengarger's laboratory identified this substance as Non Suppressible Insulin-Like Protein or NSILAp which induces insulin-like effects on various tissues. A review of the literature is presented in the light of this case.