Abstract Aims Mosaic loss of Y chromosome (LOY) in blood cells is the most common acquired mutation, increases with age, and is related to cardiovascular disease. Loss of Y chromosome induces cardiac fibrosis in murine experiments mimicking the consequences of aortic valve stenosis, the prototypical age-related disease. Cardiac fibrosis is the major determinant of mortality even after transcatheter aortic valve replacement (TAVR). It was hypothesized that LOY affects long-term outcome in men undergoing TAVR. Methods and results Using digital PCR in DNA of peripheral blood cells, LOY (Y/X ratio) was assessed by targeting a 6 bp sequence difference between AMELX and AMELY genes using TaqMan. The genetic signature of monocytes lacking the Y chromosome was deciphered by scRNAseq. In 362 men with advanced aortic valve stenosis undergoing successful TAVR, LOY ranged from −4% to 83.4%, and was >10% in 48% of patients. Three-year mortality increased with LOY. Receiver operating characteristic (ROC) curve analysis revealed an optimal cut-off of LOY >17% to predict mortality. In multivariate analysis, LOY remained a significant (P < 0.001) independent predictor of death during follow-up. scRNAseq disclosed a pro-fibrotic gene signature with LOY monocytes displaying increased expression of transforming growth factor (TGF) β-associated signaling, while expression of TGFβ-inhibiting pathways was down-regulated. Conclusion This is the first study to demonstrate that LOY in blood cells is associated with profoundly impaired long-term survival even after successful TAVR. Mechanistically, the pro-fibrotic gene signature sensitizing the patient-derived circulating LOY monocytes for the TGFβ signaling pathways supports a prominent role of cardiac fibrosis in contributing to the effects of LOY observed in men undergoing TAVR. Structured Graphical Abstract Structured Graphical Abstract Mosaic loss of Y chromosome and mortality post TAVR.