In chronic lymphocytic leukaemia (CLL), bone marrow biopsy (BM) sections show different infiltration patterns (nodular, interstitial, mixed diffuse) with a fairly homogeneous distribution when bilateral biopsies are examined. Sequential studies demonstrate that these patterns represent a dynamic process reflecting the degree of lymphocytic burden. Moreover, BM histopathology (diffuse vs non-diffuse) is a highly significant prognostic parameter. Although partly related to clinical stages, BM patterns have prognostic value on their own. A combined clinicopathologic staging system (integrating clinical stages and bone marrow histology) predicts the outcome of CLL patients more accurately than clinical stages alone.