Periodontal disease (PD) is the second most common disease affecting dogs in UK veterinary practices. Veterinary and human literature suggests that periodontal disease may be associated with bacteraemia and a chronic, systemic release of inflammatory mediators which produce direct or immune-mediated changes elsewhere in the body. Thirty canine periodontal patient’s electronic medical histories were analysed for comorbidities. The findings were analysed overall to identify any possible associations. Seventy three percent of these dogs had comorbidities, most commonly haematopoietic, cardiovascular, musculoskeletal and hepatic systems were involved. The most prevalent comorbidities were: high liver enzymes, heart murmur, mitral valve disease, and monocytosis. Other interesting comorbidities were: endocarditis, neutrophilia, submandibular lymph node enlargement and arthritis. Periodontal disease patients had a higher prevalence of disease when compared to the disease prevalence data for UK pet dogs in general. Mitral valve disease was over 17 times more likely in the periodontal disease patients. Comorbidity disease prevalence also increased with more severe periodontal disease stages. This study supports an association between periodontal disease and systemic diseases in the dog. Further studies should focus on confirming a cause and effect relationship. Until then, these data may be useful for veterinarians to examine periodontal patients for concurrent diseases and can be used as a tool to promote dental disease prevention to pet owners.