OBJECTIVE In breast cancer, diagnosis of a small internal mammary lymph node (IMLN) metastasis of less than 10 mm in size has been difficult. Our purpose was to retrospectively evaluate MRI findings of small IMLN metastases in comparison with dissected IMLNs. METHODS We studied 43 dissected IMLNs (range 2-12 mm, mean 4.512+/-2.763 mm) in 16 women with breast cancer (15 primary, and 1 recurrent). MRI examinations were performed using a 1.5 Tesla scanner (200FX; Toshiba, Tokyo, Japan) to obtain noncontrast T1-weighted SE images (TR/TE; 500/15 or 400/15ms), with a slice thickness of 5 mm on coronal images, 10 mm or 7 mm on sagittal images, FOV 15x15 cm, matrix 256x256, using a surface coil with patients in the supine position. MR images were evaluated regarding the major diameter and shape and margin of each node. RESULTS Regarding the presence of IMLN metastases, there was a significant difference between nodes with a major diameter of 5 mm or more and those of less than 5 mm (p<0.05). Using the size-based criterion (defining< or =5 mm as positive), MRI had 90.7% accuracy, 93.3% sensitivity, and 89.3% specificity. There were no significant differences in the shape-or margin-based criterion. CONCLUSIONS MRI was useful in diagnosing small IMLN metastases, using a size-based criterion.