OBJECTIVES: To describe the clinical and echocardiographic findings in dogs with quadricuspid aortic valve (QAV). BACKGROUND: QAV is a rare canine congenital heart disease which has been reported only three times in the young dog. ANIMALS, MATERIALS AND METHODS: Six dogs (0.3- to 13-year-old) with QAV diagnosed by two-dimensional echocardiography were retrospectively evaluated. Medical records, echocardiograms, and follow-ups were reviewed. RESULTS: According to aortic cusp morphology, QAV was classified as type A (n=1), type B (n=4) or type C (n=1). QAV was associated with at least one other heart disease in all of the dogs including, ventricular septal defect (n=1), enlarged left coronary ostium (n=4), degenerative mitral valve disease (MVD, n=1) and patent ductus arteriosus (PDA, n=3). Mild to moderate aortic regurgitation was also detected in all dogs by continuous-wave and color-flow Doppler echocardiography. QAV was diagnosed in four asymptomatic dogs referred for evaluation of a heart murmur. The remaining two dogs had QAV and PDA with evidence of mild exercise intolerance and moderately retarded growth. The PDA was surgically corrected in both dogs and at the time of writing, 1-2.5 years after the initial diagnosis, none of the six animals shows evidence of clinical signs. CONCLUSION: QAV is a cause of aortic insufficiency. It may incidentally be found by two-dimensional echocardiography in dogs of various ages in association with other congenital or acquired cardiac abnormalities.