About 6 years old, male Labrador retriever was presented for 10 days long weakness and fatigue. After diagnostic examinations, a preliminary diagnosis of Leptospirosis was suggested. Cardiologist consultation was advised. At admission, the dog was depressed, the visible mucosa was pale and capillary refilling time was slowed. Weak systolic murmur and rhythm disturbances were auscultated. Thoracic radiography revealed the accented bronchial pattern and non-significant peribronchial opacity. Cardiac silhouette stayed within normal limits. Two-dimensional echocardiography showed mild hypertrophy of left ventricular walls, with wall segments hypokinesia. The chambers were not dilated, there was no evidence of valves damage. Systolic function was preserved. Electrocardiography showed several sinus complexes and paroxysms of ventricular tachycardia up to 30-40 beats. Myocarditis was suspected and Bartonellosis (Bart. vinsonii&henselae), Borreliosis (Bor. burgdorferi), Troponin I tests were recommended. Amiodarone, ramipril, doxycycline, ampicillin and prednisone were prescribed and an endomyocardial biopsy was performed. Histopathological examination revealed severe myocardial degeneration and inflammatory cells infiltration. Based on positive tissue PCR Leptospirosis test and histopathology findings, excluding other possible infection, resulting in myocardium inflammation, the diagnosis of immune-mediated secondary myocarditis was made.