OBJECTIVE The surgical strategy for treatment of synchronous liver metastases from colorectal cancer remains controversial. This retrospective analysis was conducted to compare the postoperative outcome and survival of patients receiving simultaneous resection of liver metastases and primary colorectal cancer to those receiving staged resection. METHODS Between January 1988 and September 2005, 219 patients underwent liver resection for synchronous colorectal liver metastases, of whom, 40 patients received simultaneous resection of liver metastases and primary colorectal cancer, and 179 patients staged resections. Patients were identified from a prospective database, and records were retrospectively reviewed. Patient, tumor, and operative parameters were analyzed for their influence on postoperative morbidity and mortality as well as on long-term survival. RESULTS Simultaneous liver resections tend to be performed for colon primaries rather than for rectal cancer (p = 0.004) and used less extensive liver resections (p < 0.001). The postoperative morbidity was comparable between both groups, whereas the mortality was significantly higher in patients with simultaneous liver resection (p = 0.012). The mortality after simultaneous liver resection (n = 4) occurred after major hepatectomies, and three of these four patients were 70 years of age or older. There was no significant difference in long-term survival after formally curative simultaneous and staged liver resection. CONCLUSIONS Simultaneous liver and colorectal resection is as efficient as staged resections in the treatment of patients with colorectal cancer and synchronous liver metastases. To perform simultaneous resections safely a careful patient selection is necessary. The most important criteria to select patients for simultaneous liver resection are age of the patient and extent of liver resection.