Bone marrow biopsies of 720 patients with multiple myeloma (MM) were investigated from 1968-1989. Histologic variables were correlated with clinical parameters and survivals to determine prognostic factors. In 207 of these patients initial levels of serum beta 2 microglobulin (SB2M) were also measured for prognostic evaluation. Four tumour growth patterns were distinguished: interstitial (56%), interstitial/sheets (13%), nodular (15%) and packed marrow (16%) with median survivals of 46, 31, 22 and 16 months. When grouped according to the tumour cell mass in the biopsy, four histologic stages were recognized. Cellular characteristics were used to classify MM into 6 histologic types which were subsequently combined into 3 grades of malignancy: low, intermediate and high, analogous to the malignant lymphomas. With respect to tumour products, only SB2M proved to be a valuable prognostic indicator for staging and follow-up. Complications of MM such as anaemia, azotaemia, osteolytic lesions, hypercalcaemia and hypoalbuminaemia all predicted a poor prognosis, highly significant in the test statistics. We propose a new prognostic approach in MM, comprising 1) parameters defining the tumour itself, 2) the tumour products (SB2M) and 3) the tumour complications. This prognostic strategy combines information both on stage and malignancy of MM and enables definition of smouldering and of aggressive variants of MM at an early stage.