Platelets are cells that circulate in the blood and function in blood clotting. Unusually large platelets and slightly low platelet counts have been reported in Cavalier King Charles Spaniels (CKCS), estimated to affect up to 30 percent of the breed. However, whether these abnormalities are inherited or pose a problem in the normal functioning of this breed’s platelets are unknown. Although inherited platelet disorders are uncommon, several disorders in other breeds have been well documented. Studying and documenting the platelet biology in these breeds through different platelet studies has helped owners, breeders and veterinarians understand more about prevention, treatment and screening for these disorders. The purpose of this study is to be the first to describe the number, function and structure of platelets in CKCS. Tests that help describe these things include the following: platelet count, platelet volume, platelet aggregation and electron microscopy of platelets. In this way, we can then see if there is an association between a platelet disorder and another very common problem in the CKCS – mitral valve disease (MVD). Interestingly, an association between MVD and a platelet disorder has recently been described in related human beings.