Bone marrow biopsies of 137 patients with Waldenström's macroglobulinaemia (WM), 26 with non-secretory immunocytoma and 32 with benign monoclonal gammopathy were processed for histologic evaluation. Bone marrow involvement was found in 110 (80%) initially, and in 24 (18%) in sequential biopsies. 3 types were distinguished: lymphoplasmacytoid (47%), lymphoplasmacytic (42%) and polymorphous (11%) with median survivals of 74, 25 and 12 months, respectively. When grouped according to the tumour cell mass in the biopsies, the median survivals were 55, 21 and 8 months for less than 20 vol%, 20-50 vol% and greater than 50 vol% respectively; in each subtype, the tumour cell mass correlated with the disease progression. 6 clinical variables were also found prognostically significant. These results demonstrate that (i) 98% of patients with WM have bone marrow involvement; (ii) the lymph node sub-classification is applicable to the bone marrow and has both clinical and prognostic significance; (iii) patients may be staged according to the tumour cell burden in the bone marrow biopsy.