Nine patients with solitary plasmacytoma of the spine were reviewed. Four of these patients progressed to multiple myeloma within 9 +/- 4 months (mean +/- SD) from diagnosis, and died from their disease in 23 +/- 15 months. In contrast to this, the five remaining patients free of systemic disease or local recurrence survived 78 +/- 66 months. No correlation was found between age at diagnosis, lesion location, symptomatology, laboratory studies, surgical treatment, or radiation dosage and progression to systemic disease. Solitary plasmacytoma will progress to systemic multiple myeloma and death in approximately 50% of patients. The remaining patients exhibit long-term survival following local treatment of the solitary lesion.