Extramedullary plasmacytomas (EMP) rarely occur during the course of multiple myeloma (MM). Most frequent reported sites are superior respiratory airways, pleura, lung, lymph nodes, skin, subcutaneous and soft tissues, testicles and liver. EMP involving the urinary tract are very uncommon and have been ill-described in the literature. We report two unusual cases of obstructive urinary tract EMP revealing a relapse of MM after allogeneic stem cell transplantation. Clinicians must be aware that EMP may be responsible for urinary tract obstruction even in the absence of medullary progression of MM.