Lymph node metastases are important determinants in the prognosis of primary colorectal cancer. Although it has been established that enlarged, palpable lymph nodes contain metastases in less than half of the cases, no definitive data concerning the incidence of metastases in lymph nodes measuring 5 mm or less are available. We treated the surgical specimens of 52 consecutive patients who had colon cancer with a lymph node clearance technique at the Roswell Park Memorial Institute, Buffalo. We found 2699 lymph nodes in the 52 specimens, with a mean of 52 lymph nodes per specimen (range, five to 151). Sixty-four lymph nodes were found with metastases in 21 (40%) of the 52 patients. Fifty-nine of 64 of the lymph nodes were reexamined and remeasured. Thirty-nine lymph node metastases measured less than 5 mm, 13 were between 5 and 10 mm, and eight were larger than 10 mm. We concluded that lymph node metastases in colon cancer occur most frequently in lymph nodes measuring less than 5 mm (small lymph nodes). The use of lymph node clearing techniques in surgical specimens improves detection of small lymph node metastases and thereby diminishes understaging.