Background: Multiple myeloma is a common hematologic malignancy consistently preceded by monoclonal gammopathy of undetermined significance (MGUS). Little is known about postdiagnosis clinical predictors of progression of MGUS to multiple myeloma to guide MGUS management. This study aimed to investigate whether the rate of rise in serum monoclonal protein concentration during the year after MGUS diagnosis-M-protein velocity-predicts progression of MGUS to multiple myeloma. Methods: Data from the U.S. Veterans Health Administration system were used. A retrospective cohort of patients with MGUS who progressed to multiple myeloma were matched on age at MGUS diagnosis and race in a 1:4 ratio to the patients with MGUS using incidence density sampling. Kaplan-Meier curves were plotted. Univariable and multivariable conditional logistic regression analyses were fitted from the matched risk sets. Results: A total of 128 cases and 490 matched controls were included. The case group contained a higher percentage of patients with M-protein velocity >0.1 g/dL/year than the control group (44.5% vs. 28.2%, P <0.0001). M-protein velocity of >0.1 g/dL during the year following MGUS diagnosis was positively associated with progression of MGUS to multiple myeloma (multivariable-adjusted odds ratio ¼ 2.15; 95% confidence interval, 1.37-3.35). Conclusions: Patients with a positive M-protein velocity during the year after MGUS diagnosis may be considered for more frequent monitoring for early detection and timely treatment of multiple myeloma. Future prevention studies could target these patients for intervention evaluation. Impact: Our results suggest a new clinical predictor of progression to multiple myeloma following MGUS diagnosis, which has potential to identify high-risk patients for management and prevention.