Three small breed dogs weighing less 5 kg were admitted with dyspnea and cardiac murmurs. One dog had primum type ASD and chylothorax was concurrently noted. Two dogs had secundum type ASD with and without concomitant pulmonic stenosis. Although medical treatment was attempted, two dogs with secundum type ASD died and a dog with primum type ASD was successfully managed with medication. Defects were confirmed by the postmortem examination in two cases. We first describe the comparison of clinical and diagnostic features in different anatomic, hemodynamic, and pathophysiological types of ASD in small breed dogs and they show some differences with ASD in large breed dogs.