Internal mammary lymph nodes (IMLNs) metastasis is of great significance for patients with breast cancer (BC), but the diagnosis of metastasis is difficult. The purpose of this study was to explore the characteristics of metastatic internal mammary lymph nodes visualized on breast ultrasound (US) in preoperative breast cancer. Between March 2014 and May 2020, a total of 278 patients with primary BC were enrolled in a retrospective study and were divided into a metastatic group (n=224) and a non-metastatic group (n=54) according to IMLN status. Medical records, US findings, and especially IMLN status (long and short diameter, cortical thickness, blood flow) were reviewed, analyzed, and correlated with pathologic results. There were significant differences in long diameter, short diameter, numbers, intercostal space (ICS) distribution, and structure type of IMLN between the two groups (P<0.05), but no statistical difference in the ratio of long to short diameter and blood flow (P>0.05). The best cutoff values of size for differentiating IMLN metastasis from benign LNs were 10.5 mm (long diameter), 4.5 mm (short diameter), and 1.9 mm (cortical thickness), with sensitivities of 62.9%, 71.4%, and 91.4%, respectively and specificities of 90.7%, 77.8%, and 86.7%, respectively. The sensitivity and specificity of long and short diameter combined with structure type of IMLN were 74.1% and 83.3%, respectively. US is an important tool to evaluate the status of IMLN in patients with BC. US features of metastatic IMLNs include thickened cortex (≥1.9 mm), absent fatty hilum, multiple (n≥2). The size (long or short diameter) combined with structural types of IMLN had good efficacy for diagnosing IMLN metastasis. 2020 Gland Surgery. All rights reserved.