Infection by Brucella is characterized by the inhibition of host immune responses. MicroRNA-155 (miR-155) has been implicated in the immune response to many diseases. In this study, miR-155 expression during Brucella 16M infection of macrophages and mice were analyzed. Expression of miR-155 was significantly induced in macrophages at 24 hours post infection. Analysis of infected mice showed that miR-155 was inhibited at 7 and 14 days, but induced at 28 days. Very interestingly, the induction or inhibition trend was reversed at 7 and 14 days in 16M△virB-infected mice. This suggested that decreased expression of miR-155 at an early stage of infection was dependent on intracellular replication. In humans with brucellosis, serum levels of miR-155 were significantly decreased compared to those without brucellosis and healthy volunteers. Significant correlations were observed between serum level of miR-155 and serum anti-Brucella antibody titers and symptom of sweat. The decrease in miR-155 with Brucella infection contrasts with the increase in miR-155 observed in Mycobacterium tuberculosis infection. This contrasting effect suggests that Brucella interferes with miR-155-regulated immune responses through a unique mechanism. Taken together, data from this study indicate that Brucella infection affects miR-155 expression, and that human brucellosis patients show decreased serum levels of miR-155.