It has been recognized that monoclonal gammopathy of undetermined significance (MGUS) precedes a diagnosis of plasma cell myeloma in most patients. Recent gene expression array analysis has revealed that an MYC activation signature is detected in plasma cell myeloma but not in MGUS. In this study, we performed immunohistochemical studies using membrane CD138 and nuclear MYC double staining on bone marrow biopsies from patients who met the diagnostic criteria of plasma cell myeloma or MGUS. Our study demonstrated nuclear MYC expression in CD138-positive plasma cells in 22 of 26 (84%) plasma cell myeloma samples and in none of the 29 bone marrow samples from patients with MGUS. In addition, our data on the follow-up biopsies from plasma cell myeloma patients with high MYC expression demonstrated that evaluation of MYC expression in plasma cells can be useful in detecting residual disease. We also demonstrated that plasma cells gained MYC expression in 5 of 8 patients (62.5%) when progressing from MGUS to plasma cell myeloma. Analysis of additional lymphomas with plasmacytic differentiation, including lymphoplasmacytic lymphoma, marginal zone lymphoma, and plasmablastic lymphoma, reveals that MYC detection can be a useful tool in the diagnosis of plasma cell myeloma.