In a series of 172 patients with non-Hodgkin's lymphoma (NHL) classified according to the Working Formulation (WF) the overall incidence of bone marrow infiltration (BM+) at diagnosis was 39%: 59% for low-grade (LGML), 30% for intermediate-grade (IGML), and 25% for high-grade malignant lymphomas (HGML). The features most significantly correlated with the presence of BM+ were a low grade of histological malignancy, the degree of splenomegaly and high values of LDH, while those correlated with the extent of BM+ were a non-focal pattern of BM disease, the presence of blood involvement at diagnosis, and the degree of BM fibrosis. Blood involvement was detected at diagnosis in 13% of patients, and a further 16% developed a leukemic phase during the course of the disease. Blood involvement correlated significantly with splenomegaly, bulky disease, advanced clinical stage, and extent of BM+. The presence of BM infiltration 'per se' at diagnosis did not significantly affect prognosis. However, the extent of BM disease was correlated with a poorer outcome in IGML and HGML patients. Regarding peripheral blood involvement, in LGML patients only late leukemic conversions were significantly associated with a worse prognosis. In patients with IGML and HGML, either initial or subsequent blood involvement was correlated with significantly poorer outcome.