The presence of lymphoid nodules in bone marrow biopsy was investigated at diagnosis in 200 patients with chronic myeloproliferative disorders (MPD). Twelve out of 51 patients with idiopathic myelofibrosis (IM) showed such a feature (23.5%), versus two out of 100 with Ph1-positive chronic myeloid leukaemia, two of 32 with polycythaemia vera, and one of 17 with essential thrombocythaemia, the difference between IM and the remaining MPD being statistically significant (P less than 0.0001). When IM patients were compared for their initial characteristics according to the presence or not of bone marrow lymphoid nodules, patients with such a histological finding showed significantly lower values for either WBC counts, number of primitive cells in the blood, and serum lactic dehydrogenase levels. Moreover, it was observed that virtually all patients with lymphoid nodules were in the nonmyelosclerotic phases of IM. Finally, among the 14 of 32 IM patients (44%) investigated for circulating immune complexes who gave a positive test, a significant association between this immunological abnormality and bone marrow lymphoid nodules was found. The above results reinforce the immunological significance of the finding of bone marrow lymphoid nodules in IM and give support to the hypothesis of an immune component in the pathogenesis of the disorder.