Human herpes virus-8 (HHV8)-associated multicentric Castleman disease (MCD) is an unusual systemic lymphoid hyperplasia induced by HHV8-infected B cells. Most cases develop in the background of human immunodeficiency virus (HIV) infection. Despite the haematological problems at presentation and the difficulties in the initial diagnosis, the bone marrow appearances of MCD have not been described. In this study we examined the pathology of bone marrow in MCD with a view to identify the features that may be helpful in early diagnosis. Bone marrow aspirates and biopsies from 13 cases of MCD (11 of which were HIV+) and 66 control bone marrow biopsies from HIV-infected cases were studied. The specimens were routinely processed and stained. Immunohistochemistry for HHV8, immunoglobulin light chains, B-cell and plasma-cell markers was performed. The most important features were the presence of characteristic MCD lymphoid follicles containing HHV8+ plasmablasts in three of 13 cases of MCD and scattered interstitial HHV8+ plasmablasts in 11 of 13 cases. In control cases, no such follicles were seen and interstitial HHV8+ plasmablasts were rarely detected (four of 66 cases). Our results suggest that the presence of HHV8+ plasmablasts within lymphoid follicles and/or interstitium of the bone marrow are helpful features for the early diagnosis of MCD.