An EDTA-anticoagulated blood sample collected from a 1.5-year-old, intact male, English Bulldog was submitted for a CBC. The CBC data and blood smear evaluation revealed borderline high hematocrit (54%, reference interval 37–55%), inappropriate rubricytosis, moderate leukopenia due to both mature neutropenia and lymphopenia, and mild thrombocytopenia. Numerous leukocytes showed evidence of karyolysis, pyknosis, and karyorhexis, and apoptotic bodies were frequent in the background. Many neutrophils had botryoid nuclei characterized by increased numbers of nuclear segments radially arranged with spoke-like, delicate chromatin filaments connecting the segments centrally. The finding of botryoid nuclei and inappropriate rubricytosis was indicative of severe hyperthermia, such as heatstroke. The dog had been exercised a long time during conditions of high temperature and humidity until he collapsed. The dog was diagnosed with severe heatstroke, hypovolemic shock, disseminated intravascular coagulation, and multiorgan dysfunction syndrome. Despite aggressive treatment, the patient died of cardiopulmonary arrest. Botryoid nuclei are frequent in people with heatstroke. In the authors' experience, botryoid nuclei are seen commonly in dogs with heatstroke, but they have never been reported in veterinary medicine. The presence of petechiation with only mild thrombocytopenia and inappropriate rubricytosis also is suggestive of heatstroke and manifests ongoing life-threatening vascular derangement.