We reviewed the clinical and morphological findings in 43 cases of monoclonal gammopathy of undetermined significance (MGUS), 9 of smoldering multiple myeloma (SMM), and 23 of overt multiple myeloma (MM). In all cases, the patients' physicians had requested a bone marrow examination because of the possibility of MM. In all 75 cases, 3H-thymidine labeling indices were performed. The plasma cell labeling index correctly classified 62 of the 75 cases (83%). A linear discriminant function combining the labeling index and percentage of plasma cells improved the accuracy to 92% (69/75), or to 95% (71/75) if patients in whom MM developed within 6 mo were considered to have MM. The labeling index was most critical for the differential diagnosis of MM from SMM (p less than 0.001). Serum or urine M-protein level, percentage of plasma cells or lymphocytes in the bone marrow, and plasma cell grade, asynchrony, and nucleolar size failed to discriminate the group with SMM from the group with MM. In patients with MGUS or SMM, a plasma cell labeling index greater than 0.4% warned of impending MM. The plasma cell labeling index is a reliable diagnostic test when applied in cases of monoclonal gammopathy, especially when differentiation from MM is difficult using standard clinical criteria.