AimsCoronary telangiectasia (CT) is a rare congenital anomaly causing ventricular shunt and myocardial ischaemia. Its prevalence, genetic background, and impact in human hypertrophic cardiomyopathy (HCM) are unknown and were therefore investigated in this study.Methods and resultsAmong 445 patients with HCM, 195 had a coronary angiography and 124 a left ventricular endomyocardial biopsy. CT draining into the ventricular cavities was observed in 5 of 195 HCM patients (2.5%), whereas it was detected in 0.1% of 1000 consecutive subjects without congenital anomalies undergoing coronary angiography. Patients with CT‐HCM underwent a total body computed tomography scan to investigate the presence of systemic vascular malformations. HCM‐related MYH7, MYBPC3, TNNT2, and TPM1 genes and hereditary haemorragic telangiectasia‐related endoglin and activin receptor‐like kinase 1 genes were analysed. Histology, clinical profile, and outcome of CT‐HCM patients were correlated with those of 22 control HCM patients. No mucocutaneous or systemic vascular malformations were detected. Gene analysis showed a MYH7 mutation in two patients, with an associated endoglin point mutation. Histology showed in the CT‐HCM cohort a more pronounced myocardial fibrosis (29.8 ± 3.8%) compared with HCM controls (13 ± 2.6%), and disorganized cardiomyocytes separated by thin‐walled large vessels adherent to the endocardium. Clinically, the CT‐HCM cohort had a higher arrhythmic profile at diagnosis and increased incidence of implantable cardioverter defibrillator (ICD) implantations and arrhythmic deaths during a long‐term follow‐up.ConclusionCT is detectable in 2.5% of HCM patients vs. 0.1% of the general population; it may derive from a co‐existing endoglin gene mutation and cause a prominent, potentially arrhythmogenic myocardial fibrosis.