Mitral valve insufficiency is the most common heart disease in the dog. The mitral valve incompetence caused by valvular degeneration can lead to a progressive increasing heart beat frequency and in some cases to congestive heart insufficiency. A 13 year old crossbred dog, with history of diagnosis and medication for heart disease at a veterinary clinic in Lima was brought to the clinic of the Veterinary Faculty (FMV), San Marcos University with severe respiratory distress and nasal secretions. The emergency treatmentconsisted of administration of electrolytes fluid (dextrose and Ringer's lactate) and furosemide. The dog returned to the FMV clinic six days later with cough. The clinical examination revealed mild to moderate rales and mitral murmurs. Then, ultrasound, blood count, urea, creatinine and echocardiography were conducted. The most revealing finding was the thickening of the mitral valve.