The authors measured the T1 and T2 relaxation times of freshly excised human mediastinal lymph nodes to determine whether the times are clinically useful in distinguishing benign from malignant nodes. All measurements were performed at 20 MHz and 40 degrees C, within 45 minutes of lymph node excision. Mean T1 and T2 relaxation times of 99 benign nodes were 566 msec (standard deviation [SD], 117 msec) and 92 msec (SD, 29 msec), respectively. For the 16 malignant nodes, these times were 640 msec (SD, 138 msec) and 105 msec (SD, 26 msec), respectively (P less than .05 for difference in T1 times, P greater than .05 for difference in T2 times). Histograms showed considerable overlap in the relaxation times of benign and malignant nodes such that absolute measurement of these times will likely be of limited clinical value.