Summary In 54 patients with multiple myeloma plasma cell infiltration was compared in bone marrow biopsies and aspirates. In 48% of cases plasma cell infiltration was comparable, in 48% infiltration in the aspirate was lower than in the biopsy. In only two cases more plasma cells were found in the aspirate. Eleven patients (20%) had less than 20% plasma cells in the aspirate and more than 50% in the biopsy. Underestimation of plasma cell load especially seems to occur in patients with a focal growth pattern of multiple myeloma or when strong fibrosis is present. 69% of patients with stage III. according to Durie & Salmon (1975), and 76% of patients with a high β2-microglobulin had more than 50% plasma cells in the biopsy, indicating that these parameters, which are based on tumour load, are influenced by other factors as well.

The bone marrow biopsy is of superior value for direct estimation of the tumour load in multiple myeloma compared to bone marrow aspirates. A prospective study is needed to determine its prognostic significance.