Over a period of 5 years, 10 pure-bred Boxer puppies, 9-16 weeks old, were presented with a history of sudden death and were diagnosed with pyogranulomatous myocarditis. The myocarditis was characterized by a mixed infiltrate composed predominantly of neutrophils and macrophages. In our retrospective study, original case records and archived materials were examined. All dogs were positive for Borrelia burgdorferi on immunohistochemistry (IHC). There was no evidence of infectious agents in formalin-fixed, paraffin-embedded (FFPE) heart tissue sections stained with hematoxylin and eosin, Ziehl-Neelsen, Gram, Grocott methenamine silver, Warthin-Starry, Von Kossa, and Steiner-Chapman stains. IHC for Chlamydia sp., Toxoplasma gondii, Neospora caninum, West Nile virus, and canine parvovirus also yielded a negative result in all dogs. Polymerase chain reaction testing for vector-borne pathogens on heart tissue from 9 of the dogs (1 frozen and 8 FFPE samples) yielded positive results for 1 dog with B. burgdorferi as well as Anaplasma phagocytophilum in another dog. Subsequently, 2 additional cases were found in a French Bulldog and a French Bulldog-Beagle mix that had identical morphology, test results, age, and seasonality to these 10 Boxer dogs. The similarities in the seasonality, signalment of the affected dogs, and the gross and microscopic lesions suggest a common etiology. Positive IHC and morphologic similarities to human Lyme carditis indicate that B. burgdorferi is likely the agent involved. An additional consideration for these cases is the possibility of a breed-specific autoimmune myocarditis or potential predisposition for cardiopathogenic agents in young Boxers.