Background: The use of open-ended questions supplementing static questionnaires with closed questions may facilitate the recognition of symptoms and toxicities. The open-ended ‘Write In three Symptoms/Problems (WISP)’ instrument permits patients to report additional symptoms/problems not covered by selected EORTC questionnaires. We evaluated the acceptability and usefulness of WISP with cancer patients receiving active and palliative care/treatment in Austria, Chile, France, Jordan, the Netherlands, Norway, Spain and the United Kingdom. Methods: We conducted a literature search on validated instruments for cancer patients including open-ended questions and analyzing their responses. WISP was translated into eight languages and pilot tested. WISP translations were pre-tested together with EORTC QLQ-C30, QLQ-C15-PAL and relevant modules, followed by patient interviews to evaluate their understanding about WISP. Proportions were used to summarize patient responses obtained from interviews and WISP. Results: From the seven instruments identified in the literature, only the free text collected from the PRO-CTAE has been analyzed previously. In our study, 161 cancer patients participated in the pre-testing and interviews (50% in active treatment). Qualitative interviews showed high acceptability of WISP. Among the 295 symptoms/problems reported using WISP, skin problems, sore mouth and bleeding were more prevalent in patients in active treatment, whereas numbness/tingling, dry mouth and existential problems were more prevalent in patients in palliative care/treatment. Conclusions: The EORTC WISP instrument was found to be acceptable and useful for symptom assessment in cancer patients. WISP improves the identification of symptoms/problems not assessed by cancer-generic questionnaires and therefore, we recommend its use alongside the EORTC questionnaires.