Rapid outgrowth of metastases after removal of the primary tumor has been described in several mouse models. Loss of primary tumor-induced inhibition of angiogenesis in the metastases has been suggested as the underlying cause. Accordingly, we recently demonstrated that vascular density in human colorectal liver metastases increases after primary tumor resection. Here, we investigate whether this increase in vascular density has, in its turn, effects on the tumor growth of the liver metastases. We analyzed tumor growth in synchronous liver metastases from patients with the primary tumor in place, in synchronous metastases from patients with the primary tumor resected and in metachronous metastases. Tumor growth was studied by assessing the percentage of cells undergoing apoptosis by activated caspase-3 staining, and the percentage of proliferating cells by Ki-67 staining. While the percentage of proliferating cells within the metastases showed a modest increase after primary tumor resection, a significant decrease in the percentage of apoptotic cells was observed. Taken together, an increased net tumor growth of the metastases occurred after primary tumor resection. This acceleration of tumor growth could be confirmed by studying biopsies taken from the same patient before and after tumor resection. Our data show that in human cancer patients, a primary tumor may inhibit the growth of its liver metastases.