Abstract: Objective：To investigate the role of the expression and methylation of micorRNA-335 (miR-335) in the diagnosis of multiple myeloma (MM). Methods：The expression level and the promoter methylation status of miR-335 in the peripheral blood mononuclear cells (PBMC) from 43 MM patients and 30 healthy controls were detected by SYBR Green I QF-PCR and methylation specific PCR (MSP), respectively, and their correlations with serum IgG, LDH, β2-M and λ chain levels were analyzed. Results：The relative expression level of miR-335 in PBMC from MM patients ［34.8(15.3, 84.9)］ was significantly lower than that from healthy controls［269.4（63.3, 780.2）, Z=4.384,P<0.05］. Among 43 MM patients, there were 11 (25.6%) with miR 335 promoter completely methylated, while 23 (76.7%, 23/30) healthy controls showed completely unmethylated (Uc=6.62, P<0.05). The relative expression level of miR-335 in first visit patients was significantly higher than that in relapse or progressed patients (Z=2.309, P<0.05). However, the promoter methylation level of miR-335 in first visit patients (25%) was significantly lower than that in relapse or progressed patients (75%, T=24.5, P<0.05). In addition, the relative expression level of miR-335 in MM patients was correlated with λ chain concentration in serum (r=0.51, P<0.05). Conclusion：The expression and methylation levels of miR-335 may be helpful to the diagnosis of MM.