Aberrant expression of microRNAs (miRNAs) has been associated with clinical outcome in patients with chronic lymphocytic leukemia (CLL). To identify a powerful and easily assessable miRNA bio-marker of prognosis and survival, we performed quantitative reverse-transcription polymerase chain reaction (qRT-PCR) profiling in 104 CLL patients with a well-defined chromosome 17p status, and we validated our findings with miRNA microarray data from an independent cohort of 80 patients. We found that miR-15a, miR-21, miR-34a, miR-155, and miR-181b were differentially expressed between CLLs with chromosome 17p deletion and CLLs with normal 17p and normal karyotype, and that miR-181b was down-regulated in therapy-refractory cases. miR-21 expression levels were significantly higher in patients with poor prognosis and predicted overall survival (OS), and miR-181b expression levels significantly predicted treatment-free survival. We developed a 21FK score (miR-21 qRT-PCR, fluorescence in situ hybridization, Karyotype) to stratify patients according to OS and found that patients with a low score had a significantly longer OS time. When we evaluated the relative power of the 21FK score with the most used prognostic factors, the score was the most significant in both CLL cohorts. We conclude that the 21FK score represents a useful tool for distinguishing between good-prognosis and poor-prognosis CLL patients.