In this study we have investigated 313 bone marrow biopsies from 280 patients with lymphoproliferative disorders. Trephines were sectioned transversely to obtain one cylinder for cryostat sectioning and immunostaining and a second for histomorphological evaluation using a plastic-embedding technique. The results obtained by histomorphological and immunohistological evaluation were compared for their contribution to staging and classification. Using both techniques, bone marrow involvement was seen in 3/43 (7.0%) biopsies from patients with Hodgkin's disease and in 193/270 (71.5%) cases with non-Hodgkin's lymphoma, including multiple myeloma and acute lymphocytic leukaemia. Immunohistology proved superior in detecting minimal mainly interstitial bone marrow infiltration in 15 leukaemia/lymphoma cases. Biopsies showing infiltration with both methods (n = 157) were re-examined for classification of lymphomatous infiltrates. Whereas immunohistology did not provide additional information in cases with Hodgkin's disease and myeloma, this method was crucial for establishing the definitive diagnosis in a number of cases with acute lymphocytic leukaemia and non-Hodgkin's lymphoma. In all of six leukaemia cases, in which no or inadequate material was available for immunophenotyping of cell suspensions, immunohistology clearly defined the subtype. In the 140 cases of non-Hodgkin's lymphoma the majority of cases (76.4%) were identically classified. In some cases, with important prognostic and therapeutic implications, immunohistology alone provided the definitive diagnosis: T-cell lymphoma (n = 2), hairy cell leukaemia (n = 2) and centrocytic non-Hodgkin's lymphoma (n = 3). Bone marrow immunohistology is, therefore, an important supplement for classical lymphoma/leukaemia diagnosis. The differences observed between histomorphology and immunohistology emphasize the importance of lymph node biopsy in lymphoma classification.