The objectives of this study were to expand on recent observations that have suggested decreased thrombopoietin receptor (c‐Mpl) expression in megakaryocytes of patients with polycythemia vera (PV) and agnogenic myeloid metaplasia (AMM). We applied an immunoperoxidase method with anti‐c‐Mpl antibody to 55 bone marrow sections from previously untreated patients with chronic myeloproliferative disorder (CMPD) or myelodysplastic syndrome (MDS). These included 8 patients with PV, 15 with AMM, 9 with essential thrombocythemia, 5 with chronic myelocytic leukemia, 9 with the 5q− syndrome and 9 with MDS with fibrosis. The findings were compared with those in four patients with reactive erythrocytosis (RE), six with immune thrombocytopenic purpura (ITP) and five normal controls. Staining intensity (SI) was moderate to strong both in normal controls and in patients with RE or ITP. In contrast, SI was weak in variable proportions of the megakaryocytes in every one of the aforementioned clonal myeloid disorders. The staining pattern (SP) was relatively uniform in MDS and heterogeneous in CMPD. Neither SI nor SP was significantly correlated with certain clinical or laboratory parameters. We concluded that altered megakaryocyte c‐Mpl expression may be a nonspecific phenomenon in various subtypes of both CMPD and MDS. However, the characteristic staining patterns may complementthe morphological distinction between clonal and reactivemyeloproliferation.