The Swedish Twin Registry, a Research Database, in the Era of New Swedish Legislation

The Swedish Twin Registry (STR), established in the late 1950s, is one of the world's most comprehensive twin registries and a cornerstone for research on genetic and environmental determinants of health. STR includes data on Swedish-born twins since 1886, complemented by longitudinal questionnaires, clinical measures, and biobank samples. In 2019, STR became a national research infrastructure with a mission to provide broader data sharing, while in 2025, STR further transitioned into a legally defined role as a 'certain research database' under the new Swedish Act on Certain Research Databases. The new legislation provides a framework for collecting and storing personal data with broad consent for future research projects, without requiring ethical approval for the data collection per se. Instead, ethical review is conducted separately for each research project seeking to use STR data. This article aims to describe STR's current role, governance, and legal context, highlighting implications for data access and biobank integration. The registry currently holds data on more than 160,000 twins, with ongoing ascertainment at 9 months and 9 years with follow-ups at ages 15, 18, and 24. Annual linkages to national health registers enrich longitudinal analyses of disease outcomes. A variable search engine is openly available via STR's web portal while higher level metadata is provided through Swedish research data services. STR's evolution illustrates both opportunities and challenges in balancing open science practices, legal compliance, and participant integrity within large-scale research infrastructures.