Various hematologic malignancies and solid tumors are increasingly diagnosed in patients with human immunodeficiency virus (HIV) infection and may be the presenting manifestation of acquired immunodeficiency syndrome (AIDS). Multiple myeloma, however, has never been reported as the presenting manifestation of AIDS. We report on a 34-year-old man who presented with back pain, paresthesias, paraparesis, vertebral bony disease, and an associated soft tissue mass. Biopsy of the mass revealed immature plasmacytes with very faint cytoplasmic expression of kappa light chains. Bone marrow biopsy revealed 25% infiltration with poorly characterized malignant cells and 15% polyclonal plasma cells. Immunofixation of serum and urine was positive for IgG kappa and kappa light chains, respectively. A bone survey revealed lesions in the skull, left femur bone, and the pelvis. The diagnosis of an anaplastic myeloma was made. Because of the poorly characterized nature of the malignant cells and the difficulties in immunophenotyping, serologic evaluation for HIV was undertaken and was positive. The concept of myeloma as an opportunistic neoplasm defining AIDS was considered. We discuss this view and recommend that patients with multiple myeloma with poorly characterized myeloma cells as well as difficulties in immunophenotyping should undergo testing for HIV infection.