Abstract Background Bespoke self-report resource-use measures (RUMs) are commonly developed or adapted for each new randomised controlled trial. Consequently, RUMs lack standardisation and validation is rarely conducted. A new generic RUM, ModRUM, has been developed using a rigorous process, including consultation with health economists and patients. ModRUM includes a concise core healthcare module, designed to be included in all trials, and depth-adding questions, which can replace or be added to core questions as needed. Modules covering other sectors are under development. The aim of this study was to test the acceptability, feasibility, and criterion and construct validity of the healthcare module of ModRUM. Methods Patients who had a recent appointment at their GP practice were invited to complete ModRUM (core module or core module with depth questions), a characteristics form and the EQ-5D-5L. Acceptability was assessed via response rates and questionnaire completion time. Feasibility was assessed by reviewing issues observed in participants’ responses and question completion rates. Construct validity was tested via hypothesis testing and known-group analyses, using Wilcoxon rank-sum and Kruskal–Wallis tests, and a generalised linear model. Criterion validity was tested by comparing ModRUM results with primary care medical records. Sensitivity, specificity, and agreement using Lin’s concordance correlation coefficient (p_c) were estimated. Results One hundred patients participated from five GP practices in the South-West of England. Acceptability was higher for the core module (20% versus 10% response rate). Question completion rates were high across both versions (> 90%). Some support was observed for construct validity, with results suggesting that healthcare costs differ dependent on the number of long-term conditions (p < 0.05) and are negatively associated with health-related quality of life (p < 0.01). Sensitivity was high for all questions (> 0.83), while specificity varied (0.33–0.88). There was a good level of agreement for GP contacts and costs, and prescribed medication costs (p_c > 0.6). Conclusion This study provided preliminary evidence of the acceptability, feasibility, and criterion and construct validity of ModRUM. Further testing is required within trials and with groups that were less well represented in this study.