Background Adverse drug events (ADEs) occur frequently in oncology and justify continuous assessment and monitoring. There are several methods for detecting them, but the trigger tool method seems the most appropriate. Although a generic tool exists, its use for ADEs in oncology has not been convincing. The development of a focused version is therefore necessary. Objective To provide an oncology-focused trigger tool that evaluates the prevalence, harm, and preventability in a standardised method for pragmatic use in ADE surveillance. Setting Hospitals with cancer care in France. Method The tool has been constructed in two steps: (1) constitution of an oncology-centred list of ADEs; 30 pharmacists/practitioners in cancer care from nine hospitals selected a list of ADEs using a method of agreement adapted from the RAND/UCLA Appropriateness Method; and (2) construction of three standardised dimensions for the characterisation of each ADE (including causality, severity, and preventability). Main outcome measure The main outcome measure was validation of the tool, including preventability criteria. Results The tool is composed of a final list of 15 ADEs. For each ADE, a 'reviewer form' has been designed and validated by the panel. It comprises (1) the trigger(s), (2) flowcharts to guide the reviewer, (3) criteria for grading harm, and (4) a standardised assessment of preventability with 6-14 closed sentences for each ADE in terms of therapeutic management and/or prevention of side-effects. Conclusion A complete 'ready-to-use' tool for ADE monitoring in oncology has been developed that allows the assessment of three standardised dimensions.