A 12-year-old spayed female Pomeranian (weighing 2.4 kg) was referred with primary complaints of acute dyspnea, cough, and lethargy. Diagnostic imaging studies found degenerative mitral valve cusps, chordae tendinae rupture, severe mitral regurgitation (5.45 m/s of peak velocity), and marked left atrial and ventricular dilation. The dog was diagnosed as having degenerative mitral valve disease (DMVD) with ISACHC stage IIIa heart failure. Her clinical condition was stabilized after administration of cardiac medication (e.g. diuretics and pimobendan). Ten months later, the dog was referred back to the clinic due to a sudden worsening of clinical signs. Echocardiographic study found pulmonary hypertension in addition to DMVD. After medication was adjusted, clinical signs were stabilized in 2 weeks. The patient was returned after 4 months for cardiac recheck and there was no obvious worsening of clinical signs. Incidental finding of a left-to-right atrial septal defect from rupture of the atrial septum secondary to marked left atrial dilation by DMVD was noted by echocardiography. To diminish left atrial volume overload, the frequencies of both furosemide and pimobendan were increased (i.e. from q 12 hr to q 8 hr) in addition to adding spironolactone (1 mg/kg q 12 hr). Based on diagnostic findings, this case was re-diagnosed as acquired atrial septal defect secondary to rupture of the atrial septum with advanced stage DMVD. The dog was then stabilized and is currently being regularly monitored.