The assessment of proliferative activity of plasma cells in multiple myeloma has prognostic significance. The percentage of the proliferating plasma cells (plasma cell labelling index--PCLI) at the time of the diagnosis correlates with survival, and is increased during transition from the plateau phase to the relapse. Flow cytometry and immunofluorescence examination of bone marrow aspirates are used for the assessment of the PCLI. This study describes a method for evaluation of PCLI in formalin-fixed, paraffin embedded material. Bone marrow biopsies from 31 patients with relapsing (n = 10) and newly diagnosed (n = 21) multiple myeloma were examined by double immunohistochemical staining: anti-CD 138 (Syndecan 1) and anti--Ki-67. The percentage of plasma cells positive for Ki-67 were counted by an image analysis system. New cases of multiple myeloma showed 0.7 to 12.7% positivity of Ki-67 labelled plasma cells (median 4.75), the relapsing cases showed 4.4 to 22.4% positivity (median 7.75). Statistic analysis revealed prognostic significance (p = 0.046). This study presents a new method for assessment of proliferative activity of plasma cells, which can be used on archived formalin-fixed, paraffin embedded material.