B-cell chronic lymphocytic leukemia (CLL) is the most common human leukemia and dysregulation of the T-cell leukemia/lymphoma 1 (TCL1) oncogene is a contributing event in the pathogenesis of the aggressive form of this disease based on transgenic mouse studies. To determine a role of microRNAs on the pathogenesis of the aggressive form of CLL we studied regulation of TCL1 expression in CLL by microRNAs. We identified miR-3676 as a regulator of TCL1 expression. We demonstrated that miR-3676 targets three consecutive 28-bp repeats within 3'UTR of TCL1 and showed that miR-3676 is a powerful inhibitor of TCL1. We further showed that miR-3676 expression is significantly down-regulated in four groups of CLL carrying the 11q deletions, 13q deletions, 17p deletions, or a normal karyotype compared with normal CD19(+) cord blood and peripheral blood B cells. In addition, the sequencing of 539 CLL samples revealed five germ-line mutations in six samples (1%) in miR-3676. Two of these mutations were loss-of-function mutations. Because miR-3676 is located at 17p13, only 500-kb centromeric of tumor protein p53 (Tp53), and is codeleted with Tp53, we propose that loss of miR-3676 causes high levels of TCL1 expression contributing to CLL progression.