Atrial septal defect (ASD) is a common congenital heart disease (CHD) in humans, but considered relatively rare in veterinary medicine. However, modern echocardiographic and Doppler techniques currently offer a good view of the morphology of the interatrial septum, thus facilitating earlier detection of ASD in awake animals. In this context, we carried out a retrospective study of cases of recently diagnosed ASD in dogs and cats at the Cardiology Unit of Alfort (2001-2005) using echocardiography combined with colour Doppler mode. ASD was diagnosed in 156 animals and represented 37.7% of all canine and feline CHDs (n=414). ASD was the most common CHD after mitral dysplasia in both species. Boxer and Domestic shorthair were the most common canine and feline breeds affected. Most defects (98.7%) were secundum-type ASD, without clinical signs in 73.7% of cases. The most common clinical signs included systolic murmur heard over the left heart base (20.2%), exercise intolerance (7.0%), syncope (5.3%), dyspnoea (2.6%) and cough (2.6%). Animals that presented a systolic heart murmur over the left base had a significantly larger ASD than others (P<0.05). These data suggest that the incidence of ASD is higher than previously assumed. ASD should be suspected, for example, in instances of left basal systolic heart murmur, although its clinical and haemodynamic consequences are usually minor.