Background and objective: Along with impaired aerobic capacity, increased slope of the relationship between ventilation (V E ) and pulmonary CO 2 output (VCO 2 ), i.e., V E -VCO 2 slope is a common finding in patients with cardiac amyloidosis (CA), which suggests ventilatory inefficiency. Little is known about mechanisms leading to ventilatory inefficiency in CA patients. The purpose of this investigation was to examine the factors that underlie the abnormal ventilatory efficiency in transthyretin hereditary CA patients, such as excessive ventilatory drive, inability of pulmonary blood flow to increase adequately during exercise and excessive sympathetic stimulation, which are known mechanisms of V E -VCO 2 slope increase. Methods: In this single-center retrospective observational study, consecutive patients ( n = 41) with known familial transthyretin amyloidosis p .Val142Ile mutation carriers with confirmed cardiac phenotype were included. Results: Compared with CA patients without ventilatory inefficiency (VE-VCO2 slope < 36), patients with ventilatory inefficiency (VE-VCO2 slope ≥ 36) had increased inter-ventricular septum thickness, lower VO 2 peak along with hyperventilation, and prolonged post-exercise heart rate recovery. By multivariate analysis, only excess of minute-ventilation at anaerobic threshold ( β = 0.127; p = 0.011) remained an independent predictor of ventilatory inefficiency. Conclusion: Our data suggest that high ventilatory stimulation during exercise leading to hyperventilation is the main determinant of ventilatory inefficiency in hereditary transthyretin cardiac amyloidosis patients. This novel finding helps to better understand the mechanism of exercise intolerance in these patients where physiological limitation may be related to both heart dysfunction and abnormal pulmonary response.