AIM: To describe the distribution of micrometastases in the surrounding liver of patients with primary liver cancer (PLC), and to describe the minimal length of resection margin (RM) for hepatectomy.METHODS: From November 2001 to March 2003, 120histologically verfied PLC patients without macroscopic tumor thrombi or macrosatellites or extrahepatic metastases underwent curative hepatectomy. Six hundreds and twenty-nine routine pathological sections from these patients were re-examined retrospectively by light microscopy. In the prospective study, curative hepatectomy was performed from November 2001 to March 2003 for 76 histologically verfied PLC patients without definite macroscopic tumor thrombi or macrosatellites or extrahepatic metastases in preoperative imaging. Six hundreds and forty-five pathological sections from these patients were examined by light microscopy. The resected liver specimens were minutely examined to measure the resection margin and to detect the number of daughter tumor nodules,dominant lesions, and macroscopic tumor thrombi inside the lumens of the major venous system. The paraffin sections were microscopically examined to detect the microsatellites, microscopic tumor thrombi, fibrosis tumor capsules, as well as capsule invasion and the distance of histological spread of the micrometastases.RESULTS: In the retrospective study, 70 micrometastases were found in surrounding liver in 26 of the 120 cases (21.7%). The farthest distance of histological micrometastasis was 3.5 mm, 5.3 mm and 6.0 mm in 95%, 99% and 100% cases, respectively. Macroscopic tumor thrombi or macrosatellites were observed in 18 of 76 cases, and 149 micrometastases were found in the surrounding live in 25 (43.1%) of 58 cases with no macroscopic tumor thrombi. The farthest distance of histological micrometastasis was 4.5 mm, 5.5 mm and 6.0 mm in 95%, 99% and 100% cases, respectively.Two hundred and sixty-seven micrometastases were found in surrounding liver in 14 (77.8%) out of 18 cases with macroscopic tumor thrombi or macrosatellites. The farthest distance of histological micrometastasis was 18.5 mm, 18.5 mm and 19.0 mm in 95%, 99% and 100% cases, respectively.CONCLUSION: The required minimal length of RM is 5.5 mm and 6 mm respectively to achieve 99% and 100% micrometastasis clearance in surrounding liver of PLC patients without macroscopic tumor thrombi or macrosatellites, and should be greater than 18.5 mm to obtain 99% micrometastasis clearance in surrounding liver of patients with macroscopic tumor thrombi or macrosatellites.