By lymphoid myelofibrosis or hairy cell leukaemia or tricholeukaemia is meant an unusual haemopathic condition known only for the past few years. It is characterized pathognomonically by the presence of lymphocyte type cells with villous extroflexions, hence the name "hairy cell". Clinically the disease presents as an involutive myelopathy associated with splenomegaly, generally without any particular lymph gland involvement. The attention of students today is concentrated on the nature of the hairy cells; while some are inclined to admit their monocyte or histiocyte derivation, others consider that they derive from B lymphocytes. Therapeutically, almost everybody agrees that splenectomy is the only valid step. A case of H.C.L., which was typical from the clinical and laboratory viewpoints is reported. It is probable that certain haemopathic pictures once classified among atypical leucoses and lymphomas, would today be more correctly classed as hairy cell leukaemia.