A 5 year-old spayed female Cavalier King Charles Spaniel was presented after a 3- to 5-day onset of severe respiratory distress. The dog also had a history of several episodes of syncope prior to presentation. A comprehensive diagnostic investigation revealed a midsystolic click sound on cardiac auscultation, signs of left sided cardiac enlargement in ECG and thoracic radiography, mitral valvular leaflet protrusion into left the atrium, decreased E-point-to septal separation (EPSS) and mitral regurgitated flow in echocardiography, all of which are characteristic signs of mitral valvular prolapse. After intensive care with antidiuretics and a vasodilator with oxygen supplement, the condition of the dog was stabilized. The dog was then released and is being medicated with angiotensin converting enzyme (ACE) inhibitor with regular follow-up.