Between 1960 and 1985, 30 patients with solitary plasmacytomas were treated with radiotherapy at the University of Iowa: 13 patients with extramedullary plasmacytomas (EMP) and 17 with solitary plasmacytomas of bone (SPB). The local control rates were 92% for patients with EMP and 88% for those with SPB. Two of nine patients (22%) with EMP treated to the primary tumor only developed regional lymph node metastasis, indicating the need for elective irradiation of this area. The most common pattern of failure in both groups was progression to multiple myeloma. This occurred in 23% of the patients with EMP and 53% of those with SPB. The time course of progression to multiple myeloma differed for the two groups. All of those who progressed to multiple myeloma in the EMP group did so within 2 years, whereas a significant number of those in the SPB group progressed more than 5 years after initial therapy. None of five patients who received adjuvant chemotherapy in the SPB group progressed to multiple myeloma, compared to 75% (9/12) of the patients who did not receive chemotherapy.