The adrenal origin of hypercorticism is established by usual hormonal and morphological investigations. Adrenal causes of Cushing syndrome are mainly due to unilateral tumours. In some cases, the remaining difficulty is evaluation of prognosis. Here, the contribution of molecular biology is likely to be major. To better understand the mechanisms of adrenal tumourigenesis, it seems essential to seek the factors modulating steroidogenesis and adrenal cells proliferation.