665 Background: While considered safe, simultaneous resection of colorectal cancer primary and liver metastases is not performed routinely. We aimed to identify practice patterns, short and long-term outcomes of simultaneous vs. staged resections for synchronous colorectal cancer liver metastases. Methods: We conducted a population-based cohort study of patients undergoing resection for synchronous colorectal cancer liver metastases from 2006-2015 by linking administrative healthcare datasets in Ontario, Canada. Resection of the primary colorectal cancer and liver metastases within six months was considered synchronous disease. Simultaneous (same hospital admission) and staged resections were compared. Outcomes were 90-day post-operative mortality, total length of hospital stay, overall survival and healthcare costs. Survival for the staged group was measured from the last surgical resection to death and estimated using Kaplan Meier. Cost analysis was undertaken from the perspective of a third-party payer and compared using t test. Results: Of 2,738 patients undergoing colorectal and liver resection for colorectal cancer, 1,168 were synchronous, of which, 442 underwent simultaneous resection. The rate of simultaneous resections increased on average by 3% per year (p = 0.02). Median total length of stay was shorter (8 vs. 11 days); 90-day post-operative mortality was higher for simultaneous resections (3.4% vs 1.2%). Median overall survival was worse with simultaneous resection (40 months, 95%CI 35-46 vs. 78 months, 95%CI 59-86), with a 5-year overall survival of 37% (simultaneous) and 55% (staged). Mean overall costs were lower for simultaneous resections ($12,722 CAD vs. $16,455 CAD. Conclusions: Simultaneous resection compared to staged resection for patients with synchronous colorectal cancer liver metastases is associated with higher 90-day postoperative mortality and worse survival. It was associated with shorter length of hospital stay and lower costs for the health cares system. Considering selection bias, randomized studies would be necessary to determine the role of simultaneous resection for synchronous disease colorectal cancer liver metastases.