A 5-month-old Maltese was examined because of a holosystolic heart murmur. Results of echocardiography were suggestive of a small isolated interventricular septal defect. Color flow and pulsed-wave spectral Doppler echocardiography confirmed that there was left-to-right blood flow through the defect during systole and diastole. Because of the small size of the defect, the large systolic pressure differential between the ventricles (72.6 mm Hg), and the lack of clinical signs, the small amount of left-to-right shunting was considered clinically unimportant, and no medication or treatment was recommended. Seven months later, the dog was re-examined, and trans-septal blood flow was no longer seen. Isolated interventricular septal defects are a common congenital heart disorder in some breeds of dogs. Such defects may be subclinical in some dogs. In others, they cause a wide spectrum of clinical problems. Findings in this dog suggest that interventricular septal defects may close spontaneously in some dogs.