The aim of this study was to recognise the etiological factor of a disease with symptoms of lameness and cardiac failure, which occurred in one dog 4 weeks after invasion by ticks. A serological examination as well as molecular examination (PCR) was done. In the sample of the serum, the presence of antibodies specific to Borrelia burgdorferi were detected. Antibiotic therapy with doxycycline did not cause significant improvement, so the owners of the dog decided about its euthanasia. During the necroscopy, a dilated heart was recognised. In the heart samples, the genetic material of Borrelia was detected. The results of serological and molecular examinations showed that in the discussed case, an etiological factor of the disease was spirochetes. In light of the research, veterinary practitioners should keep in mind the presence of Lyme disease in dogs in Poland and include it in differential diagnoses for lameness and cardiological problems.