The authors report 40 cases of functionless pituitary adenomas, investigated by the same protocol and treated in the same way, between 1976 and 1982. The clinical, biological, neuroradiological features, clinical course and anatomopathological findings of this very uniform series are described. A complete immuno-histochemical study was undertaken. These findings suggest that the diagnosis of a true functionless pituitary adenoma should be based on the following criteria: --clinical and biological: absence of hormonal hypersecretion; --histological: absence or very few signs of secretory activity (little ergastoplasma, few mitochondria and Golgi apparatus); --immuno-histochemical: absence of reactivity to immunofluorescence and immuno-peroxydase methods.