BACKGROUND: The impact of new-onset left bundle branch block (N-LBBB) developing after Transcatheter Aortic Valve Replacement (TAVR) on cardiac function and mechanical dyssynchrony is not well defined. METHODS: We retrospectively screened all patients who underwent TAVR in our centre between Oct 2018 and Sept 2021 (n = 409). We identified 38 patients with N-LBBB post-operatively (of which 28 were persistent and 10 were transient), and 17 patients with chronic pre-existent LBBB (C-LBBB). We excluded patients requiring pacing post TAVR. For all groups, we retrospectively analysed stored echocardiograms at 3 time points: before TAVR (T0), early after TAVR (T1, 1.2 ± 1.1 days), and late follow-up (T2, 1.5 ± 0.8 years), comparing LV mass and volumes, indices of LV function (LV ejection fraction, LVEF; global longitudinal strain, GLS), and mechanical dyssynchrony indices (systolic stretch index, severity of septal flash). RESULTS: At baseline (T0), C-LBBB had worse cardiac function, and larger LV volumes and LV mass, compared with patients with N-LBBB. At T1, N-LBBB resulted in mild dyssynchrony and decreased LVEF and GLS. Dyssynchrony progressed at T2 in persistent N-LBBB but not C-LBBB. In both groups however, LVEF remained stable at T2, although individual response was variable. Patients with better LVEF at baseline demonstrated a higher proportion of developing LBBB-induced LV dysfunction at T2. Lack of improvement of LVEF immediately after TAVR predicted deteriorating LVEF at T2. In transient LBBB, cardiac function and most dyssynchrony indices returned to baseline. CONCLUSIONS: N-LBBB after TAVR results in an immediate reduction of cardiac function, in spite of only mild dyssynchrony. When LBBB persists, patients with better cardiac function before TAVR are more likely to have LBBB-induced LV dysfunction after TAVR.