UNLABELLED This retrospective study was designed to evaluate serum levels of intact parathyroid hormone and calcium in patients with conotruncal heart defects with or without microdeletion 22q11.2 in order to investigate a correlation between various types of conotruncal heart defect and hypoparathyroidism. A total of 67 patients with truncus arteriosus, tetralogy of Fallot, pulmonary atresia and ventricular septal defect, interrupted aortic arch or vascular ring were included of whom 28 had a 22q11.2 deletion (Group I) and 39 did not (Group II). In two patients of Group I and in one patient of Group II, parathyroid hormone level was decreased with normal serum calcium levels. No patient of Group II showed hypocalcaemia. In Group I, complete hypoparathyroidism with low parathyroid hormone and hypocalcaemia occurred in seven patients; 5 patients had bilateral anomalies of the right and the left 4th aortic arch derivates. In addition to an interrupted aortic arch type B or a high aortic arch, the contralateral subclavian artery arose cervically, high thoracically or anomalously from the descending aorta. Two patients had unilateral anomalies of the 4th aortic arch system: The origin of the right subclavian artery was cervical or from the descending aorta. CONCLUSION Hypoparathyroidism occurred in 7 of our 28 patients with conotruncal heart defect and monosomy 22q11.2 and was associated with an extended regression of the 4th aortic arch development on both sides of the aortic arch system.