IgM myeloma is a very rare and poorly defined entity. In a detailed assessment of 10 cases, it was demonstrated that 70% had an aberrant phenotype based on the expression of CD19, CD45, CD27 and Cyclin D1 but all cases lacked CD56 and CD117. Interphase fluorescence in situ hybridization demonstrated deletion 13 in 50% while 5/8 cases assessed had a t(11;14). Despite the high incidence of the t(11;14), CD20 was only expressed in one of nine cases. We conclude that IgM myeloma is a distinctive subset characterized by a CD20−CD56−CD117− phenotype and the t(11;14).