Lymph node and bone marrow trephine biopsies of seventy two consecutive cases of non-Hodgkin's lymphoma occurring among black Zimbabweans were reviewed to determine bone marrow involvement. The bone marrow was involved in 23.6% of patients but the proportion of bone marrow positive cases was highest in low grade lymphomas, though these were the least common type of lymphoma encountered. Cells infiltrating the marrow showed high degree of concordance with corresponding lymph node histology. Low grade lymphomas (CLL excluded) had a predominantly diffuse pattern of marrow involvement while intermediate and high grade lymphomas had nearly equal proportions with diffuse and focal patterns. Bone-marrow involvement did not make a significant difference to the staging of non-Hodgkin's lymphoma as most cases not involving the marrow also presented with advanced disease. Mean age of patients was negatively correlated with histological grade but was not related to bone marrow involvement. Central nervous system involvement occurred mostly in the high grade lymphomas and was proportionately distributed between bone marrow positive and negative cases. 40% of non-Hodgkin's lymphoma were associated with HIV infection but HIV-associated lymphoma surprisingly very rarely infiltrated the bone marrow.