Serum beta2-microglobulin (Sbeta2M), serum albumin, platelet count, serum creatinine, and age emerged as powerful predictors of survival and were then used in the tree analysis approach. A combination of Sbeta2M and serum albumin provided the simplest, most powerful and reproducible three-stage classification. This new International Staging System (ISS) was validated in the remaining patients and consists of the following stages: stage I, Sbeta2M less than 3.5 mg/L plus serum albumin > or = 3.5 g/dL (median survival, 62 months); stage II, neither stage I nor III (median survival, 44 months); and stage III, Sbeta2M > or = 5.5 mg/L (median survival, 29 months). The ISS system was further validated by demonstrating effectiveness in patients in North America, Europe, and Asia; in patients less than and > or = 65 years of age; in patients with standard therapy or autotransplantation; and in comparison with the Durie/Salmon staging system. CONCLUSION) The new ISS is simple, based on easy to use variables (Sbeta2M and serum albumin), and recommended for early adoption and widespread use.