Noncoding RNAs play a pivotal role in the pathogenesis of chronic lymphocytic leukemia (CLL). We hypothesized that microRNAs (miRs) are involved in the transition from monoclonal B-cell lymphocytosis (MBL) to CLL and tested miR-15a/16-1 cluster, miR-21, and miR-155 expression in purified B cells of normal individuals, individuals with MBL, and patients with CLL. When we analyzed 224 samples from 2 independent training and validation cohorts, we found that miR-155 was overexpressed in B cells from individuals with MBL, and even more so in B cells from patients with CLL, when compared with B cells from normal individuals. Furthermore, we were able to identify miR-155 in circulating microvesicles from both individuals with MBL and patients with CLL. Next, to examine the prognostic role of miR-155, we measured its expression level in plasma samples collected before treatment initiation in 228 patients with CLL. We found significantly higher miR-155 expression levels in patients who failed to achieve a complete response compared with those who experienced complete response. Our findings support the use of cellular and plasma levels of miR-155 as biomarkers for the risk of progression in individuals with MBL, as well as to identify patients with CLL who may not respond well to therapy.