The Diseases Database is a free website that provides information about the relationships between medical conditions, symptoms, and medications. The database is run by Medical Object Oriented Software Enterprises Ltd, a company based in London.

The site's stated aim is "education, background reading and general interest" with an intended audience "physicians, other clinical healthcare workers and students of these professions". The editor of the site is stated as Malcolm H Duncan, a UK qualified medical doctor.

The Diseases Database is based on a collection of about 8,500 concepts, called "items", related to human medicine including diseases, drugs, symptoms, physical signs and abnormal laboratory results.

In order to link items to both each other and external information resources three sets of metadata are modelled within the database.^{[citation needed]}

The UMLS map also enables links to and from other medical classifications and terminologies e.g. ICD-9 and SNOMED.