From June 2 to 4, 2025, the Sandpit “Building SHARE: the Synthetic Health dAta REpository” took place in Frankfurt am Main on the campus of the University Hospital. The aim of the event was to develop innovative solutions for building an international synthetic health data repository (SHARE) that facilitates research and development in the field of artificial intelligence (AI) and data-driven science in particular. The focus was on the use of rare diseases as a use case, as real patient data is often scarce in this area and synthetic data opens up new possibilities. The Sandpit provided an experimental space in which experts from different disciplines – including patient advocacy groups, AI and machine learning researchers, computer scientists, data management experts and clinicians – worked together to identify key challenges, develop solutions and define concrete next steps for future collaboration.