AIM:To compare the safety and efficacy of simultaneous versus two stage resection of primary colorectal tumors and liver metastases. METHODS:From January 1996 to May 2004, 103 colorectal tumor patients presented with synchronous liver metastases. Twenty five underwent simultaneous colorectal and liver surgery and 78 underwent liver surgery 1-3 mo after primary colorectal tumor resection. Data were retrospectively analyzed to assess and compare the morbidity and mortality between the surgical strategies. The two groups were comparable regarding the age and sex distribution, the types of liver resection and stage of primary tumors, as well as the number and size of liver metastases. RESULTS:In two-stage procedures more transfusions were required (4 ± 1.5 vs 2 ± 1.8, pRBCs, P < 0.05).Chest infection was increased after the two-stage approach (26% vs 17%, P < 0.05). The two-stage procedure was also associated with longer hospitalization (20 ± 8 vs 12 ± 6 d, P < 0.05). Five year survival in both groups was similar (28% vs 31%). No hospital mortality occurred in our series. CONCLUSION:Synchronous colorectal liver metastases can be safely treated simultaneously with the primary tumor. Liver resection should be prioritized over colon