ABSTRACT:Two dogs were presented owing to sudden rapid growth of cutaneous masses greater than 10 cm on the neck and axillary region, respectively. Based on history and results of physical examinations, blood work, fine needle aspiration, histopathological examination, and computed tomography, inoperable grade III infiltrating mast cell tumours were diagnosed. After the initiation of imatinib treatment, the masses markedly shrank and became undetectable within 10 days in both dogs, although none of the tumour specimens showed evidence of mutations in sequencing of c-kit exons 8 and 11. These results suggest that imatinib could be a therapeutic option in patients with surgically inoperable canine mast cell tumours, even those that are histopathologically high grade without c-kit exon 8 or 11 mutations.