Development of the Nursing Minimum Data Set for the Netherlands (NMDSN): identification of categories and items Rationale Currently, there is no systematic collection of nursing care data in the Netherlands, while pressure is growing from the profession, policy-makers and society to justify the contribution of nursing and its costs. A nursing minimum data set can provide data to demonstrate nursing's contribution to health care as it can be used to describe the diversity of different patient populations and the variability of nursing activities, and to calculate the associated nursing workload. Objective To identify categories and items for inclusion in the Nursing Minimum Data Set for the Netherlands. Design A multimethod, exploratory approach was used. This included interviews, document analysis, consensus rounds, seeking validation in the literature, and drawing up lists of most frequently occurring patient problems, interventions and outcomes of care. Eight hospitals, with a total of 16 wards, participated in the study. Results Relevant categories and items emerged after analysis and grouping of the material and included: five hospital-related items, six patient demographics items, seven medical condition items, 10 nursing process items, 24 patient problems, 32 nursing interventions, four outcomes of nursing care, and three complexity of care items. Almost every item could be located in the existing documentation systems, the lists of patient problems, outcomes and interventions, or in the literature. Conclusion A set of categories and items of nursing data has been identified. The content validity of this set is partly supported by its consistency with the literature, findings from practice and the judgement of potential users. Nursing outcomes need further development. The data set will be tested in practice to find out whether the categories and items are useful, and whether they can be minimized.