Abdominal ultrasound is frequently performed as part of initial staging of canine mammary tumors. The aims of this study were to describe abdominal ultrasonographic findings present at diagnosis of mammary tumors in dogs and to assess their value for associations with histopathological results and treatment outcome. The medical records of 201 female dogs diagnosed with mammary tumors that had abdominal ultrasonography performed as part of their initial clinical evaluation were reviewed. Histopathological diagnosis of malignant mammary tumors was obtained in most cases (86%; 172 cases). One hundred and fifty-seven animals (78%) had some ultrasonographic abnormality. A total of 338 ultrasonographic changes were identified, and the spleen was the organ with the highest frequency of ultrasonographic changes (20%), followed by the uterus (18%) and the liver (16%). Abdominal ultrasonography abnormalities were not associated with mammary tumor malignancy (OR = 1.7; P = 0.27). Ultrasonographic evidence of enlarged intraabdominal lymph nodes was associated with metastases in surface regional lymph nodes (P