Manuscripts containing Greek medical texts were inventoried by author and work at the beginning of the 20th century by a group of philologists under the direction of Hermann Diels. Useful as it was – and will continue to be – Diels’ catalogue omitted authors and works, misidentified manuscripts, and overlooked codices. Furthermore, since the publication of the catalogue, some libraries have adopted a new system of classification, manuscripts have been destroyed, items have changed location, and new ones have come to light. The present Census is a checklist of the Greek medical manuscripts currently known in collections worldwide. It is both an amended and updated index of Diels’ catalogue, and a list of the items missed or overlooked in Diels, or located since. Although it does not supersede Diels’ catalogue, it is the indispensable instrument for a New Diels, and will be the reference for years to come for any new critical edition and medico-historical research based on manuscripts, besides providing the basis for a broad range of other historical inquiries, from codicology to the history of medicine and science, including Byzantine intellectual history, Renaissance studies and humanism, history of the book and early printing, and the history of medical philology and learning.