The relationship between the ultrastructure of plasma cells and clinical features was analyzed in 54 patients with multiple myeloma: 20 elderly patients (9,8 and 3 cases of IgG, IgA and BJP types, respectively; 5,3 and 12 cases in clinical stage I, II and III, respectively) and 34 young patients (16, 10, 1 and 7 cases of IgG, IgA, IgD and BJP types, respectively; 5, 10 and 19 cases in clinical stage I, II and III, respectively). Five-year survival rates of the elderly and young groups were 41.5 and 60.5%, respectively, though the difference of both survival curves was not significant. Response rates of initial therapy in the elderly and young groups were 55 and 56%, respectively (not significant). The number of patients presenting disarrangement of organelles was significantly larger in the elderly group, but those presenting other abnormal structures were equal between both groups. The ratios of dense body, intramitochondrial granules and multilamellar body in bone marrow plasma cells were significantly higher in nonresponders than in responders of the elderly group. The ratios of disarrangement of organelles, single sac loop-like structure and intramitochondrial granules in bone marrow plasma cells were significantly higher in nonresponders than in responders of the young group. It is suggested that the ultrastructure of plasma cells in multiple myeloma differs between elderly and young patients and is useful in predicting the respective clinical features, although drug responses and survival curves do not show a significant difference.