The trephine bone marrow biopsies of 51 patients with myeloproliferative syndromes were revised searching for lymphoid follicles and lymphoplasmocytosis: 18 of these had idiopathic myelofibrosis and 33 chronic myelogenous leukaemia. Six of the 18 biopsies on patients with myelofibrosis showed lymphoid follicles but only one of the 33 with chronic meylogenous leukaemia did (P = 0.01, Fisher exact test). In addition, four of the six myelofibrosis having follicles had two or more of them. When the pathological pattern of myelofibrosis was considered according to the Lennert and al. classification we found significantly more follicles in the cellular phase of the disease than in the advanced phases (P = 00.4, Fisher exact test). These findings can be considered as a morphological argument supporting the idea of an immunological mechanism in the development of myelofibrosis.