BACKGROUND: The application of precision medicine in oncology requires in-depth characterisation of a patient's tumours and the dynamics of their responses to treatment. PATIENTS AND METHODS: We used next-generation sequencing of circulating cell-free DNA (cfDNA) to monitor the response of a KIT p.L576P-mutant metastatic vaginal mucosal melanoma to sequential targeted, immuno- and chemotherapy. RESULTS: Despite a KIT mutation, the response to imatinib was mixed. Unfortunately, tumours were not accessible for molecular analysis. To study the mechanism underlying the mixed clinical response, we carried out whole-exome sequencing and targeted longitudinal analysis of cfDNA. This revealed two tumour subclones; one with a KIT mutation that responded to imatinib and a second KIT-wild-type subclone that did not respond to imatinib. Notably, the subclones also responded differently to immunotherapy. However, both subclones responded to carboplatin/paclitaxel, and although the KIT-wild-type subclone progressed after chemotherapy, it responded to subsequent re-administration of paclitaxel. CONCLUSION: We show that cfDNA can reveal tumour evolution and subclonal responses to therapy even when biopsies are not available.