To evaluate the most important factors in the prognosis and staging of multiple myeloma (MM), the presenting clinical features of 163 previously untreated patients with MM were correlated with survival duration using univariate and multivariate regression analyses. The univariate proportional hazard analysis ranked the parameters in the following order of importance: platelet count, hemoglobin level (Hb), tumor cell mass stage, lytic bone lesions, creatinine, and age. When the individual contribution of each variable was assessed by multivariate regression analysis, platelet count was confirmed to be the dominant feature for prognosis and clinical stage provided additional information. The introduction of platelet count could then be used to improve the reliability of the Durie and Salmon staging, by allowing to separate the high-risk group (stages II and III) into a smaller subgroup (22%) of thrombocytopenic patients (less than 150 x 10(9) platelets/L) whose risk of death was actually very high (median survival, 9 months) and a larger subgroup (46%) of patients with normal platelet count and intermediate or standard risk (median survival, 48 months). This simple change in the prognostic system gave rise to markedly different survival curves also after the exclusion of patients with renal failure and applied successfully to both old and young patients (greater than and less than 50 years, respectively). Finally, platelet count, Hb, and lytic bone lesions could be combined simply to stratify patients with normal renal function into three risk groups: (1) low (39% of cases; median survival, 79 months), (2) intermediate (53% of cases; median survival, 48 months), and (3) high (8% of cases; median survival, 19 months).