The purpose of this study was to investigate false-negative results of preoperative axillary ultrasound (US) and to evaluate clinicopathologic factors related to false-negative results in patients with invasive breast cancer. Four-hundred eighty-two patients with 483 invasive breast cancers who had no suspicious findings on preoperative axillary US were included in this study. All patients underwent sentinel lymph node biopsy (SLNB) or axillary lymph node dissection. False-negative and true-negative results were compared in terms of age, body mass index (BMI), T-stage, pathologic N-stage and final diagnosis of breast cancer. Statistical analyses were performed using the chi-square or Fisher's exact tests. Of the 483 axillae with negative results on axillary US, 93 axillae of 93 patients showed false-negative results and the negative predictive value of axillary US was 80.8% (390/483). Seventy-five axillae (15.5%, 75/483) had N1 and 18 axillae (3.7%, 18/483) had N2 or N3 disease. Eighteen false-negative results with N2/N3 disease showed a significantly higher T stage (T2/T3) than those with N1 disease. As the T-stage increased, false-negative results were found more often on preoperative axillary US (p < 0.05). Age, BMI and final diagnosis of primary breast cancer were not associated with false-negative results on preoperative axillary US. Preoperative axillary US alone is insufficiently specific to obviate the need for SLNB because of the substantial number of false-negative results in patients with invasive breast cancer, although preoperative axillary US alone may exclude most cases of N2 and N3 disease.