Objective: To evaluate the clinical outcome of subcutaneous mast cell tumors (SQMCT) and to identify clinical and histological characteristics of more aggressive disease. Study design: Retrospective study. Animals: Forty-five dogs with 48 SQMCTs. Methods: Medical records were reviewed (2011-2021) for patient information, clinical, and histopathological data including multinucleation, necrosis, invasion into local muscle, an infiltrative growth pattern, tumor grade (if listed), mitotic index, and surgical margins. The presence of local recurrence, lymph node metastasis, survival time, and other parameters evaluating patient outcome were also recorded. Results: Local recurrence occurred in 17.8% (8/45) of dogs, 11.1% (5/45) developed metastatic recurrence, and 26.7% (12/45) developed lymph node metastasis. Dogs with lymph node metastases had a median disease-free interval (DFI) of 194 days (18-1864), while median DFI was not reached for dogs without lymph node metastasis (p = .0012). Median survival time for dogs with lymph node metastasis was 551 days (110-2050) compared to 1722 days (10-1722) without metastasis (p = .0432). Local recurrence resulted in a significantly shorter median survival time of 551 days (80-2050) compared to 1722 days (10-1722) for dogs without local recurrence (p = .0038). Dogs with infiltrative tumors had a median DFI of 268 days (3-1722) and DFI for dogs without an infiltrative pattern had not reached median at 1864 days (10-1864) (p = .011). Conclusion:Lymph node metastasis decreased disease-free interval and survival. Clinical significance: Subcutaneous mast cell tumors may be a more aggressive disease than previously reported. | INTRODUCTIONMast cell neoplasia is a disease that carries a variable clinical outcome based on its histologic grade, tumor location, and other histological and clinical features and is most commonly present in the dermis.