Objective:To explore the correlation between body mass index (BMI) and the risk of multiple myeloma (MM).Methods:A two-sample Mendelian randomization (MR) analysis was utilized. The data were obtained from genome-wide association study (GWAS) datasets in the IEU OpenGWAS database. The dataset number for exposure factor BMI was ieu-a-2, involving 339 224 mixed populations (males and females) and containing 2 555 511 single nucleotide polymorphisms (SNP); the dataset number for outcome factor MM was ieu-b-4957, involving 372 617 European populations (601 MM patients and 372 016 healthy individuals), containing 8 615 746 SNP; the BMI and MM reference genomes were both HG19/GRCh37 provided by the Genetic Investigation of Anthropometric Traits (GIANT) collaborative organization. From dataset ieu-a-2, 79 SNP significantly associated with BMI were selected as instrumental variables ( F > 10), and then validated in the outcome factor dataset (ieu-b-4957), with missing SNP excluded, resulting in 76 retained instrumental variables. MR analyses were performed using the inverse variance weighted (IVW), MR-Egger regression, weighted median method, simple mode method, and weighted mode method. Sensitivity analyses were conducted using the Cochran Q test, MR-Egger regression intercept and leave-one-out approach. Results:IVW results showed a positive correlation between BMI and the risk of MM ( OR = 1.001, 95% CI: 1.000-1.002, P = 0.012), which was supported by the MR-Egger method ( OR = 1.003, 95% CI: 1.000-1.005, P = 0.022). The sensitivity analyses results showed that neither the MR-Egger intercept (intercept = -4.336×10 -5, P = 0.158) nor the heterogeneity test (IVW: Q = 82.02, P = 0.271) found significant heterogeneity or horizontal pleiotropy, indicating high robustness of the results. Conclusions:BMI may be a potential risk factor for the development of MM.