The idiotype (Id) determinants on the multiple myeloma immunoglobulin can serve as tumor-specific antigens. An anti-Id immune response may stem the growth of the malignant clone. We report on 26 patients treated at our institution with high-dose chemotherapy and peripheral blood progenitor cell transplantation (PBPCT) and vaccinated with the Id protein. The patients received chemotherapy and PBPCT to establish a minimal residual disease state. After high-dose therapy, the patients received a series of monthly immunizations consisting of 2 intravenous infusions of dendritic cells (DCs) pulsed with either Id protein or Id coupled with keyhole limpet hemocyanin (KLH) as an immunogenic carrier protein, followed by subcutaneous boosts of Id-KLH conjugates. DCs were obtained in all patients from a leukapheresis product 3 to 9 months after PBPCT. Patients were observed for toxicity, immune responses, and tumor status. The DC infusions and the administration of Id-KLH boosts were well tolerated, with patients experiencing only minor and transient side effects. Of the patients, 24 of 26 generated a KLH-specific cellular proliferative immune response. Only 4 patients developed an Id-specific proliferative immune response. Three of these immune responders were in complete remission at the time of vaccination. A total of 17 patients are alive at a median follow-up of 30 months after transplantation. Id vaccination with autologous DCs is feasible for myeloma patients after transplantation. Id-specific cellular responses can be induced in patients who are in complete remission. Further studies are needed to increase the rate of anti-Id immune responses in patients who do not achieve complete remission. Biol Blood Marrow Transplant 2000;6(6):621-7.