While quadricuspid morphology is commonly observed in truncal valves, quadricuspid aortic valves (QAV) are rare and their natural history is not well described. This retrospective study of 37 patients describes the diagnostic associations and morphologic variability of QAVs in children (median age at diagnosis 4.3 y IQR 0–12 y). Associated congenital heart diseases were present in 54% (most commonly tetralogy of Fallot (TOF) and valvar pulmonary stenosis). Among patients with isolated QAV, 11 had genetic syndrome and 5 had skeletal anomalies. Valve morphology was most commonly type B (41%) and A (35%; Hurvitz and Roberts). Dilated aortic root (Z ≥ 2) was present in 5 and dilated ascending aorta in 9 patients at diagnosis. All patients with type C (n = 3) and G (n = 1) had aortic dilation. At diagnosis, >mild AR was rare (n = 1), mild regurgitation was common (n = 12, 32%), >mild AS was rare (n = 2), and mild AS was uncommon (n = 4). Over a median follow-up of 3.3y (IQR 0.9-11y), progression of AR was seen in 2 patients and progression of aortic root or ascending aorta dilation (increase in Z score by ≥ 2) was seen in 5 patients. In conclusion, QAV is a rare congenital anomaly and about half of the cases are found in hearts that are otherwise structurally normal. A relatively high prevalence is seen in patients with TOF, pulmonary stenosis, skeletal deformities, and genetic syndromes. Meticulous evaluation of aortic valve morphology should be conducted on echocardiograms performed for these indications.