Bone marrow biopsies of 491 untreated and 170 treated patients with Hodgkin's disease (HD) were investigated. Marrow involvement was found in 10% and 25% respectively. Positive biopsies were rare in clinical stages I and II (1% and 2%), but the incidence rose to 25% and 45% in stages III and IV. HD patients with nodular sclerosis in lymph node histology had a low incidence of bone marrow involvement (4%), while those with lymphocytic depletion had a high incidence (22%). Of nine clinical and six histological parameters tested, bone marrow involvement proved to be the most significant predictive factor indicating an unfavourable course. Moreover, classification of the bone marrow manifestations according to the degree of lymphocytic infiltration, proved to be simple, reproducible and prognostically significant. Normal haematopoietic tissue was found in only 20% of the negative biopsies of untreated patients. The remaining 80% were characterized by a variety of non-specific reactions. These included marrow hypoplasia and leukaemoid and exudative reactions each of which indicated a poor prognosis; and epithelioid-cell granulomas and lymphoid nodules which predicted favourable survival curves. Adequate bone marrow biopsy is a valuable part of the investigation of patients with HD, as both positive and negative biopsies provide information of prognostic significance.