Abstract: Background: The prognostic significance of marrow involvement in diffuse large cell lymphoma (DLCL) is controversial. Factors that that have been reported to influence prognosis include the pattern and extent of marrow infiltration and histological discordance between the primary site and the bone marrow. Methods: Bone marrow biopsies from 172 patients with newly diagnosed DLCL entered in two consecutive trials of the Australasian Leukaemia and Lymphoma Group were analyzed. Progression-free (PFS) and overall survival (OS) were calculated according to the absence or presence of bone marrow involvement (BMI), the extent of lymphomatous infiltration and the presence of histological discordance between the primary site and the bone marrow. Results: Of 172 patients with DLCL accrued between 1982 and 1990, who were treated with CHOP or CHOP-like regimens, 47 (27%) demonstrated marrow involvement on examination of multiple levels. Seventy two percent (34/47) of patients had discordant marrow involvement (<50% large cells) and 28 had minimal (<10%) involvement; these latter patients with minimal marrow involvement (<10%) had similar PFS & OS to the 113 patients without involvement. Within the group of 47 patients with marrow involvement, an increasing percentage of BM involvement was significantly associated with an increasing percentage of concordant histology and a decreasing PFS & OS. Conclusions: Minimal BMI, seen in the majority of patients with DLCL with marrow infiltration, appears not to influence the PFS & OS. However, an increasing degree of marrow involvement is associated with an increasing component of large cells and a poorer prognosis in DLCL patients, independent of other risk factors.