Among plasma cell disorders, solitary plasmacytoma (solitary-plasmacytoma of bone, SPB and extramedullary plasmacytoma, EMP) is rare as compared with mulitiple myeloma (MM). Furthermore, the relationship between solitary plasmaeytoma and MM remains unclear. Between 1960 and 1994, 24 patients with SPB and 20 with EMP were treated. The criteria for diagonosis were: (1) No evidence of other lesions based on clinical and radiologic examinations; (2) Biopsy evidence of a plasma cell neoplasm; (3) Bone marrow biopsy specimen with negative findings (less than 10% plasma cell); (4) No anemia, hypercalcemia or renal involvement. The average follow-up period was 112 months (from 6 to 360 months). Fifty-four percent of patients with SPB and 40% of patients with EMP developed MM, however, there was no significant statistical difference between SPB and EMP (P <0.05). We suggested that solitary plasmacytomas be classified as two types, latent and aggressive. The former was histologically well-differentiated plasmacytomas. The latter was poorly differentiated tumors which easily progress to MM. The treatment of choice is wide excision or thorough curettage, by cryogenic necrosis with liquid nitrogen or cautery of the bony wall with phenol and the cavity filled with bone grafts or cement. All patients with apparently isolated plasmacytoma should be given if the tumor turns out to be poorly differentiated, in order to delay their progression to MM.