Trisomy 12, the third most frequent chromosomal aberration in chronic lymphocytic leukemia (CLL), confers an intermediate prognosis. In our cohort of 104 untreated patients carrying+ 12, NOTCH1 mutations occurred in 24% of cases and were associated to unmutated IGHV genes (P= 0.003) and+ 12 as a sole cytogenetic abnormality (P= 0.008). NOTCH1 mutations in+ 12 CLL associated with an approximately 2.4 fold increase in the risk of death, a significant shortening of survival (P< 0.01) and proved to be an independent predictor of survival in multivariate analysis. Analogous to+ 12 CLL with TP53 disruption or del (11q), NOTCH1 mutations in+ 12 CLL conferred a significantly worse survival compared to that of+ 12 CLL with del (13q) or+ 12 only. The overrepresentation of cell cycle/proliferation related genes of+ 12 CLL with NOTCH1 mutations suggests the biological contribution of NOTCH1 mutations to determine a poor outcome. NOTCH1 mutations refine the intermediate prognosis of+ 12 CLL.