This study aimed to investigate whether adipose tissue invasion (ATI) of cancer cells at the tumor margin influenced lymph node status and prognosis in patients with invasive ductal carcinoma of the breast. Data for 245 patients with breast cancer with marginal ATI were clinicopathologically compared with data for 65 patients without ATI. We also examined the value of the combination of ATI and peritumoral lymphatic vessel invasion (LVI). The frequency of axillary lymph node metastases was 40.7% in patients with ATI (99/243) and 11.3% in patients without ATI (7/62; P<.0001), and ATI was an independent factor influencing nodal metastasis. Patients with ATI had a poorer prognosis than patients without ATI (10-year disease-free survival, 76% and 94%, respectively; P=.0323). In addition, patients without ATI or LVI had neither lymph node metastasis (n=52) nor recurrent disease (n=53). ATI is one of the biologic indicators of tumor aggressiveness.