The increased medullar collagen deposition, histologically detectable as an augmentation of reticulin fibres, is known as myelofibrosis. Myelofibrosis, as an anatomopathological feature, is associated with well-diagnosed malignant or non malignant diseases, of haematopoietic or non-haematopoietic origin (Table 1); in these cases it is termed secondary myelofibrosis. In contrast, primary myelofibrosis, ie idiopathic myelofibrosis, constitutes a clinical entity per se and has been defined as a chronic myeloproliferative disorder. This disorder has been described under different names which referred to both its features, myelofibrosis and myeloid metaplasia. The most commonly used names for this disease are idiopathic myelofibrosis, myelofibrosis with myeloid metaplasia or agnogenic myeloid metaplasia. In this review, we will focus on the pathophysiology of primary myelofibrosis, that will be termed myelofibrosis with myeloid metaplasia (MMM).