AIMS: Fluctuations in left ventricular (LV) thresholds with cardiac resynchronization therapy (CRT) are unknown. The LV capture management (LVCM) algorithm automatically measures LV thresholds on a daily basis and offers the opportunity to analyse threshold fluctuations. METHODS AND RESULTS: A total of 282 patients implanted with a Medtronic Concerto CRT-D device were prospectively studied. Device data were collected at periodic visits, including daily thresholds from the preceding 14 days and weekly threshold ranges since implantation, acquired by the LVCM algorithm up to 12 months' follow-up. Overall, LV thresholds remained relatively stable, with 189/208 (91%) patients having a maximum increase in threshold of ≤ 1.0 V at any time between their 1 and 6 month visits and 127/135 (94%) between the 6 and 12 month visits. However, increase in threshold was significantly affected by LV threshold amplitude. Of the 170 patients with a 1 month threshold of ≤ 2.0 V, 159 (94%) had increases of <1.0 V up to their 6 month visit, whereas 8/38 (21%) patients with >2.0 V threshold had increases of >1.0 V (P = 0.01). There were no significant changes in LV threshold amplitude and fluctuation over the 12 month follow-up. CONCLUSION: For patients with low ( ≤ 2.0 V) LV thresholds, a safety margin of 1.0 V is sufficient to ensure LV capture if phrenic nerve stimulation is an issue, and may be even lower in devices with auto-adaptive capture management algorithms. However, the margin should be greater in patients with higher thresholds because of larger fluctuations. Left ventricular capture management may be particularly useful in these patients to ensure LV capture without sacrificing device longevity.