BACKGROUND Five or more colorectal liver metastases (CRLM) are considered marginally resectable and cannot be treated solely by hepatic resection (Hr). This study investigated the long-term effectiveness of surgical treatment using microwave coagulo-necrotic therapy (MCN) and/or Hr for marginally resectable or unresectable multiple CRLM. METHODS This study retrospectively analyzed 82 consecutive CRLM patients with ≥5 CRLM who underwent MCN, Hr, or both, at our institution from 1994 to 2012. Presuming all CRLM were resected curatively, virtual remnant liver volume was calculated using preoperative computed tomography or magnetic resonance imaging. Virtual remnant liver volume <30% was defined as unresectable. Patients were divided into marginally resectable (Group Y; n=29) and unresectable (Group N; n=53). Overall and recurrence-free survival were assessed. RESULTS Mean maximum tumor diameter and tumor number were 3.1 and 6.0 cm in Group Y and 3.3 and 11.3 cm in Group N. Surgical methods included MCN (n=16), MCN+Hr (n=9), and Hr (n=4) in Group Y, and MCN (n=28) and MCN+Hr (n=25) in Group N. One- and 2-year recurrence-free survival rates were 38.0% and 22.8% in Group Y, and 18.9% and 3.8% in Group N (P=0.01). However, 1-, 3-, and 5-year overall survival rates of Group N (86.8%, 44.6%, and 33.7%, respectively) were similar to those of Group Y (82.8%, 51.4%, and 33.3%, respectively; P= not significant each). CONCLUSIONS MCN may improve survival for patients with unresectable multiple CRLM, similar to that in patients with marginally resectable multiple CRLM.