Bone marrow modifications resulting from infections and systemic diseases can be studied by analysis of morphology and aetiology. Two types of lesions or modifications can be observed, those occurring in the connective tissue comprising inflammatory processes, acute and chronic, as well as immune reactions, and those involving the normal haematopoietic cell lines, with possible hyperplastic or aplastic changes in one or more cell lines. The main lesions are described (oedema, haemorrhage, necrosis, suppuration, granulomas, lymphoid nodules and hyperplasia, immunoblastic or plasmacytic hyperplasia), as well as the main aetiologies. In association, the three main haematopoietic cell lines show hyperplasia, hypoplasia, aplasia of one or all of the cell lines, sometimes with dysmyelopoiesis. The stroma and vessel reactions comprise myelofibrosis, gelatinous transformation or amyloid deposits. The methods for identifying aetiological agents are emphasized. It should also be stressed that malignant neoplasias of different types involving the bone marrow can be responsible for such inflammatory or immune reactions.