Objective: To examine the prognostic significance of disseminated tumor cells in blood and bone marrow of patients undergoing surgical resection of colorectal liver metastases. Summary Background Data: Despite curative hepatic resection of colorectal liver metastases, a high percentage of patients develop tumor recurrence. These recurrences probably originate from disseminated tumor cells released into the circulation before or during surgery. Methods: Thirty-seven patients with potentially curative (R0) resection of colorectal liver metastases were prospectively enrolled into the study. Preoperative bone marrow samples and preoperative, intraoperative, and postoperative blood samples were examined for disseminated tumor cells by CK20 RT-PCR. Results: Tumor cells were detected in preoperative blood samples in 11 of 37 (30%) patients, in intraoperative blood samples in 17 of 37 (46%) patients, and in postoperative blood samples in 8 of 37 (22%) patients. Four of 25 (16%) patients tested positive for disseminated tumor cells in bone marrow samples. Median follow-up time for all patients was 38 months (range, 10-63 months). Multivariate analysis confirmed tumor cell detection in intraoperative blood (P = 0.009) and in bone marrow samples (P = 0.013) to be independent prognostic factors of tumor relapse. Conclusions: This is the first study demonstrating that detection of hematogenous tumor cell dissemination during hepatic resection of colorectal cancer metastases predicts tumor relapse. Detection of disseminated tumor cells may help to individualize adjuvant therapy for patients with colorectal liver metastases and to develop surgical strategies to prevent intraoperative hematogenous tumor cell shedding.