OBJECTIVE: The purpose of this study was to identify the prevalence of occult nodal metastases on routine ultrasound examination of internal mammary (IM) nodal basins in patients with breast cancer. MATERIALS AND METHODS: Patients with primary breast cancer (n = 595) underwent breast ultrasound evaluation between September 1, 2011, and April 1, 2012. For all patients, ultrasound examination included a survey of the axillary, infraclavicular, IM, and supraclavicular nodal basins. Patient demographics, breast cancer histopathologic type, and grade, size, location, and presence of metastatic nodes in regional nodal basins were recorded. Fisher exact test and Wilcoxon rank test were used for statistical analysis. RESULTS: Fifty-eight of 595 (10%) patients had positive IM ultrasound finding, with eight (1.3%) patients having isolated IM involvement. Patients with positive IM ultrasound findings were statistically significantly younger than those without such findings (median age, 42 vs 57 years; p < 0.0001). Of the 58 patients with positive IM ultrasound, 29 (50%) underwent ultrasound-guided needle biopsy, which confirmed malignancy in 26 of 29 (90%) patients. Nonlateral (p < 0.001) grade 3 (p < 0.001) tumors larger than 5 cm (p < 0.0006) with the estrogen receptor-negative HER2/neu-negative subtype (p < 0.001) associated with axillary, infraclavicular, or supraclavicular metastases (p < 0.001) were more likely to be associated with positive IM ultrasound findings. IM ultrasound resulted in an N status change for 46 of 595 (8%) patients and of the overall clinical stage for 38 (6.4%) patients. CONCLUSION: IM ultrasound and ultrasound-guided fine-needle aspiration biopsy are feasible, sensitive, and specific. Application of IM ultrasound and ultrasound-guided needle biopsy in a selected subpopulation of young patients with medial or central estrogen receptor-negative HER2/neu-negative breast cancer may result in a change in clinical stage and modify the treatment plan.