Abstract: An 8-year-old female spayed Pug dog was presented for evaluation of cutaneous lesions occurring secondary to immunosuppressive treatment of presumed immune-mediated thrombocytopenia. Abnormal hematologic findings included persistent thrombocytopenia, macrothrombocytes, and variably shaped, often fusiform, blue cytoplasmic inclusions in neutrophils. May–Hegglin anomaly (MHA) was suspected based on the morphologic appearance of platelets and neutrophils. Examination of cells by transmission electron microscopy revealed normal platelet ultrastructure; neutrophil inclusions had features similar to those reported for inclusions in human MHA. Neutrophil function was within normal limits based on flow cytometric analysis. Thrombelastography indicated a prolonged clotting time (r), and PlateletMapping showed a lack of response to 2 μM ADP compared with a moderate response in the control dog. Immunocytochemical staining of blood smears using 2 commercially available antibodies against MYH9 protein (nonmuscle myosin heavy chain II) yielded negative results. However, genomic DNA sequencing analysis of the dog's MYH9 gene identified a single point mutation, resulting in substitution of lysine for glutamine at the 1841 amino acid position; this mutation is identical to one identified in people with MHA. To our knowledge, this is the first report of an MYH9 mutation in the dog. MHA-associated macrothrombocytopenia may be mistaken for immune-mediated thrombocytopenia.