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There is no consensus as to whether or not metastatic nodules in the liver should be biopsied for tumor grading in cases of neuroendocrine tumors with ‘synchronous liver metastasis’. In this study, we compared the Ki-67 labeling index between the primary tumor and synchronous liver metastasis in 30 patients, who had received simultaneous resections. Examined tumors were of the small bowel (n = 18) or pancreas (n = 12), and G1 or G2 in primary histologic grade. In 20 patients (67%), the Ki-67 index was similar between the primary tumor and liver metastasis, but 10 (33%) showed an elevation of 3.4–14.4% in the liver, which increased the tumor grade in 4 cases. The Ki-67 elevation in the liver was more common in G2 than G1 neoplasms (P= 0.002). The size, but not number, of liver metastases was significantly larger in patients with an elevated Ki-67 index (P= 0.006). Using 40 mm as a provisional cutoff for the greatest diameter of liver metastases, the positive predictive value of this discriminator for elevated Ki-67 was 56%, and the negative predictive value was 93%. In conclusion, synchronous liver metastases can yield a higher Ki-67 labeling index than primary neuroendocrine tumours, particularly when the secondary is greater than 40 mm.