Whole body MRI, including diffusion weighted imaging, has potential to unravel complex response patterns in patients with metastatic melanoma receiving immunotherapy. To minimise the effect of noise on resultant apparent diffusion coefficient (ADC) maps, images should be acquired using optimal b-values. These values have been calculated from MRI scans of 11 patients, with 99 metastases. The optimal pair of b-values is found to be 50 and 1250 s/mm2. A significant difference is reported between the distributions of the ADC values before and after treatment with immunotherapy. This provides preliminary evidence that ADC could provide a biomarker of response to immunotherapy.