Abnormal protein bands (APB) unrelated to the original monoclonal protein occasionally appear in serum immunofixation samples from patients with multiple myeloma (MM) following haematopoietic stem cell transplantation (HCT). To investigate the significance of APB, medical records and serum immunofixation patterns of 53 MM patients, who had undergone HCT (49 autologous and 4 allogeneic) at the stem cell transplantation unit of Gazi University Faculty of Medicine, were reviewed. Patients were staged according to Durie-Salmon and International staging systems (ISS) and disease response was determined according to European Bone Marrow Transplantation (EBMT) criteria. Fourteen (26.4%) of the 53 patients developed APBs after HCT. The median time for the appearance and duration of APB was 3 (range 1-24) and 5.5 (range 1.5-14) months, respectively. Probability of overall survival (OS) at the end of the follow-up was 77 and 61.4% in patients with and without APB, respectively (p = 0.334). The median duration of follow-up (767 days (range, 220-2905) vs. 726 days (range, 120-1780) p = 0.545) was not different in patients with and without APB. Probability of progression free survival (PFS) at the end of follow-up was 28.8% in patients with and 27.7% in patients without APB (p = 0.835). PFS (910 days (range 180-2905) vs. 730 days (range 90-1765) p 0.835) was longer in patients with APB, though without statistical significance. Thus, the occurrence of APB post-transplantation is not associated with any adverse long-term consequences and does not require treatment modification. Copyright (C) 2010 John Wiley & Sons, Ltd.