We have previously reported the frequent occurrence of bile duct invasion by liver metastases from colorectal cancer. We found that patients with macroscopic intrabiliary cancer growth survive longer after hepatectomy than those without this feature. In the present study, we analyzed the clinicopathological features of primary colorectal cancer showing macroscopic intrabiliary extension of liver metastases. We reviewed 217 patients who underwent initial hepatic resection for colorectal liver metastasis between 1992 and 1998, and analyzed the corresponding primary colorectal cancers clinicopathologically. Microscopic bile duct invasion was found in 89 of 217 cases (40.6%) and, of these cases, 23 (10.6%) had macroscopic intrabiliary extension. Histological sections of the corresponding primary colorectal cancer were available in eight (group A) of these 23 cases. These were compared with 20 cases, selected randomly, of colorectal cancer that did not show bile duct invasion and were diagnosed as liver metastases. These patients underwent hepatectomy during the same period as group A and were used as a control (group B). The histology of the primary tumors revealed well-differentiated adenocarcinoma in 100% of group A and in 25% of group B. The average maximum diameter of the primary tumor was 5.32 cm in group A and 3.61 cm in group B. Venous invasion was detected in 25% of group A and in 90% of group B (P < 0.01), while the incidences of lymphatic vessel invasion and lymph node metastases were similar between the groups. These data suggest that macroscopic intrabiliary extension could be a good indicator of a unique subgroup of colorectal cancers showing less aggressive features even though they develop liver metastases. Careful histological evaluation is important even for metastatic tumors.