Abstract Background We have recently demonstrated association of unexplained sudden cardiac arrest (SCA) with inheritance of catecholamine induced QT prolongation (CIQTP). Objective We here aim to describe incidence, characteristics and prognosis of this new syndrome in young patients with unexplained SCA or their relatives. Methods We reviewed the medical screening of all consecutive patients or their first-degree relatives explore from 2015 after the occurrence of a SCA before age 45. Structural heart disease or inherited arrhythmia diseases were excluded. A mental stress test was performed, as previously described, for each family members. All families with a positive mental stress were included in the study. Genetic screening was performed in at least one positive patient per family using targeted sequencing on a panel of 109 genes associated with inherited arrhythmias and cardiomyopathies. Results Among 456 patients screened (24 after SCA, 432 for familial screening) of 153 families, we identified 10 families (6.5%) with a catecholamine induced QT prolongation. No mutation was identified in these families. One hundred and ten patients were screened in CIQTP families. Thirty-four patients (30.9%) presented a CIQTP (mean age 42±20 yo, 64.7% of women). Five (14.7%) patients presented with previous symptoms (including 4 syncope and 1 SCA). Two patients (5.9%) were implanted with an ICD and eleven (32.3%) were treated with beta blocker therapy mainly because of QT prolongation >500 ms after mental stress test or previous symptoms. After a 3.6±1.8 years of follow up, no sudden cardiac death nor syncope occurred on beta blocker therapy except for one patient implanted with an ICD after a SCA. Under beta blocker treatment the patient was asymptomatic for 5 years. After a suddenly stop of the beta blocker treatment, the patient underwent VF. For 3 years now the patient is asymptomatic under beta blocker treatment. Conclusions In our experience, CIQTP families represent 6.5% of cases of unexplained SCD and suggest systematic screening with a mental stress test for family screening after the occurrence of a SCA. Beta blocker therapy is very efficient to reduce the risk of SCA. Funding Acknowledgement Type of funding sources: None.