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Objective: To determine the prognostic value of pre-treatment apparent diffusion coefficient (ADC) of colorectal liver metastases in predicting disease response, progression-free survival (PFS) and overall survival (OS). Methods: We retrospectively reviewed 102 patients who underwent pre-treatment diffusion-weighted MRI using a breath-hold (b=0, 150, 500) or a free-breathing (b=0, 50, 100, 250, 500, 750) technique. The mean ADC (b=0-500) and mean flow-insensitive ADC (ADChgh) values (breath- hold: b=150 and 500; free-breathing: b=100 and 500) of up to three hepatic lesions were evaluated in each patient. Clinical and laboratory parameters were recorded. Tumour response was assessed by Response Evaluation Criteria in Solid Tumors (RECIST) criteria at 12 weeks after treatment. Associations between tumour response, ADC values and clinical/laboratory parameters were examined by one-way analysis of variance. The relationship of ADC with PFS and OS was determined by Kaplan-Meier analysis. Results: 62 patients responded to chemotherapy at 12 weeks. The pre-treatment mean ADC and mean ADChigh were higher in the non-responding group than in the responding group (1.55 vs 1.36, p=0.033; 1.40 vs 1.16, p=0.024). However, the PFS and OS of the two groups of patients stratified by the median of mean ADC values or threshold derived by receiver operating characteristic analysis were not statis- tically significant. By multivariate Cox regression analysis, patients with ≦2 metastases and response to chemotherapy showed better PFS; white cell count ≦10 and surgical treatment were associated with better OS. Conclusion: Colorectal liver metastasis with higher pre- treatment mean ADC and mean ADChjgh was associated with poorer response to chemotherapy. However, ADC and ADChigh values did not predict the patient outcome in this study cohort. Advances in knowledge: High mean ADC values of colorectal liver metastases on pre-treatment diffusion- weighted MRI is associated with poorer response to chemotherapy. [ABSTRACT FROM AUTHOR]