PURPOSE: To establish the ultrasonographic (US) characteristics of benign versus metastatic lymph nodes. MATERIALS AND METHODS: One hundred fifty-eight axillary lymph nodes in 40 patients (age range, 31-73 years) surgically treated for breast cancer have been studied in vitro with a 7.5-MHz US probe in a water bath. The long-to-short axis ratio and the hilar and cortical characteristics were evaluated; the US findings were correlated with the histopathologic findings. To estimate the long-to-short axis ratio, all lymph nodes were measured. RESULTS: Of the 158 lymph nodes, 45 showed histopathologic evidence of metastasis; 38 of the 45 revealed US signs of malignancy. The signs that caused malignancy to be suspected were a long-to-short axis ratio of less than 1.5, absence of a hilus, and disruption of the cortical zone. The most specific sign for the diagnosis of metastasis was absence of the hilus. The increase in the long-to-short axis ratio was the finding that caused the most false-negative interpretations. Signs of malignancy were more accurate in lymph nodes 10 mm or larger than they were in lymph nodes smaller than 10 mm. CONCLUSION: Findings of in vitro US studies of axillary adenopathy provide the basis for the evaluation of lymph node metastasis in vivo before surgery, especially in those lymph nodes 10 mm or larger.