One hundred and nineteen methacrylate-embedded trephine biopsy specimens from 80 patients with Hodgkin's disease are reviewed. Although marrow involvement was found in 4% of the untreated patients, the present study was mainly concerned with the marrow unaffected by specific infiltrates and in negative specimens. The non-involved bone marrow in Hodgkin's disease invariably showed alterations of three types; stromal damage, inflammatory infiltration, and disturbed haematopoiesis. Each of these features can be found in the absence of one or both of the others. In severe examples, all three of these components usually occur simultaneously, giving histological pictures that mimic Hodgkin-specific infiltrates, sometimes in association with clinical suggestions of bone marrow involvement or replacement. But trephine biopsies of sufficiently high histological quality offer alternative explanations for the pancytopenia, for instance intramedullary phagocytosis, reactive sclerosing inflammation resembling auto-immune disorders, or a disturbance of haematopoiesis itself. This disturbance could be due to a defect inherent in the haematopoiesis associated with Hodgkin's disease, possibly predisposing for leukaemia in longterm survivors who have received chemotherapy and/or irradiation. The disturbed erythropoiesis proved to be strongly correlated with the stage of the disease at the time of biopsy. This finding could contribute to staging procedures, when laparotomy is contraindicated.