Medium, large and giant congenital melanocytic nevi (CMN) can impose a psychosocial burden on patients and families, and are associated with increased risk of developing melanoma or neurological symptoms. Lack of consensus on what outcomes to measure makes it difficult to advise patients and families about treatment and to set up best practice for CMN. Fostering consensus amongst patient representatives and professionals, we aim to develop a core outcome set (COS), i.e. the minimum set of outcomes to measure and report in care and all clinical trials of a specific health condition. We focused on the 'what to measure' aspect, the so-called core domain set (CDS), following COMET and CS-COUSIN guidelines.We conducted a systematic review to identify outcomes reported in literature. Focus groups with patient representatives identified patient-reported outcomes. All these outcomes were classified into domains. Through e-Delphi surveys, 144 stakeholders from 27 countries iteratively rated the importance of domains and outcomes. An online consensus meeting attended by seven patient representatives and seven professionals finalized the CDS.We reached consensus on six domains, four of which were applied to both care and research: 'quality of life', 'neoplasms', 'nervous system' and 'anatomy of skin'. 'Adverse events' was specific to care and 'pathology' to research.We have developed a CDS for medium-to-giant CMN. Its application in reporting CMN care and research will facilitate treatment comparisons. The next step will be to reach consensus on the specific outcomes for each of the domains and what instruments should be used to measure these domains/outcomes.