Clinical staging has been widely accepted as essential for optimal treatment of many types of cancer. Various groups of workers have investigated factors which influence prognosis in multiple myeloma. Important factors which have been indentified include the performance status, the presence or absence of renal insufficiency, the quantity of the monoclonal protein fraction in the serum, the extent of bone lesions, the serum concentration of albumin and calcium, and the hemoglobin level. Since our findings agreed with the staging, previously proposed by Salmon, this procedure was used to stage myeloma cases in a retrospective study. Survival was statistically significant shorter in stage III than in stage I and in subtype B shorter than in subtype A. In addition to the clinical findings we propose a system for the cytological and histological staging of multiple myeloma which is based on differences in maturity of myeloma cells and have tested its validity in predicting survival in a retrospective follow-up study. 202 cases of multiple myeloma have been analysed by cytological and histological methods. On the basis of the findings the following types were distinguished: 1. plasmocytic myeloma (127 cases), 2. plasmoblastic-plasmocytic myeloma (35 cases), and 3. plasmoblastic myeloma (32 cases). In 8 cases predominance of giant cells were seen. In types 2 and 3 involvement of extraskeletal sites (lymph node, liver, spleen) was significantly higher than in type 1, just as survival was significantly higher (39,7 months) in this type than in type 3 (9,8 months). There seemed to be no correlation between morphological type and class specificity of monoclonal immunoglobulins. Use of the clinical and morphological staging system should provide better initial assessment and follow-up of individual patients, and should lead to improved study design and analysis in large clinical trials of diagnosis and therapy for multiple myeloma.