Abstract
<jats:p>Congenital macrothrombocytopenia is a hereditary platelet disorder predominantly observed in Cavalier King Charles Spaniels. It is characterized by persistent thrombo-cytopenia accompanied by increased platelet volume (mean platelet volume, MPV), often identified incidentally in clinically healthy dogs. This case report describes an 8-month-old female Cavalier King Charles Spaniels presented for routine follow-up with no clinical signs of illness. Serial complete blood counts revealed persistent thrombocytopenia (42–55 ×10³/µL) with elevated MPV (12–14 fL). Peripheral blood smears confirmed the presence of large, young platelets. Infectious, immune-mediated, and neoplastic causes were excluded. The dog remained clinically healthy without pharmacological treatment and continues under monitoring, with preventive advice to avoid trauma-induced bleeding. Recognition of breed-specific hematological traits is essential to prevent misdiagnosis and unnecessary interventions.</jats:p>