BACKGROUND The surgical strategy for treatment of synchronous colorectal liver metastases remains controversial. The outcome and overall survival of patients presenting with such metastases, treated either by simultaneous resection or by delayed resection, were evaluated. METHODS From 1987 to 2000, 97 patients presented with synchronous colorectal liver metastases, of whom 35 (36 per cent) underwent a simultaneous resection and 62 patients (64 per cent) a delayed resection. Simultaneous resection was considered prospectively for patients with fewer than four unilobar metastases. RESULTS Age, blood transfusion requirements, operating time, duration of inflow occlusion, hospital stay and mortality rate were similar in the two groups. The morbidity rate did not differ significantly (23 per cent after simultaneous resection and 32 per cent after delayed resection). The location of the primary tumour and extent of liver resection did not influence the morbidity rate significantly in the simultaneous resection group. The overall survival rate was 94, 45 and 21 per cent at 1, 3 and 5 years respectively after simultaneous resection, and 92, 45 and 22 per cent after delayed resection. CONCLUSIONS In selected patients, simultaneous resection of the colorectal primary tumour and liver metastases does not increase mortality or morbidity rates compared with delayed resection, even if a left colectomy and/or a major hepatectomy are required.