Two patients with a history of multiple myeloma experienced a recurrence of the disease.18F-FDG PET/CT revealed prominent extramedullary disease as well as multi-foci in the bone marrow, both with increased FDG uptake. However, on 68Ga-Pentixafor PET/CT, all the myeloma lesions showed significantly lower tracer uptake in comparison with 18F-FDG PET. This false-negative result of recurrent multiple myeloma with extramedullary disease may be a potential limitation of 68Ga-Pentixafor in assessing multiple myeloma.