ICD-11 bans the letters I and O so nobody mistakes them for digits
The WHO's newest disease classification, ICD-11, took effect on 1 January 2022 after more than a decade of work by over 300 specialists from 55 countries. It holds about 85,000 entities, lets a single disease sit under two parents behind the scenes, and even keeps the letters I and O out of its codes.
Development began in 2007. Thirty working groups took part, and roughly 10,000 further proposals arrived from around the world. Testers saw an alpha in May 2011, then a beta a year later in May 2012, and the first stable release came on 18 June 2018 before WHO members endorsed it at the 72nd World Health Assembly. Like its predecessor it is revised every year; the release of 16 February 2026 was its eighth update and ninth stable version.
Underneath sits the Foundation Component, a web of more than a hundred thousand entities in which one condition can belong to several groups at once. Pneumonia, for instance, is filed both as a lung infection, by site, and as an infectious disease, by cause. Statistics need tidier lists, so the WHO derives linearizations in which each item has exactly one parent. The main one, the Mortality and Morbidity Statistics list or MMS, is what most people mean by ICD-11. It has 28 chapters, uses grey nodes to show overlaps, and adds residual Other specified and Unspecified categories, displayed in maroon in the browser and lacking Foundation IDs.
Codes were redesigned too. In ICD-10 a code was a letter plus two digits, so some bulky chapters spilled across several letters. Now every chapter gets its own first character: digits 1 to 9 for the first nine, and letters A to X for the other nineteen. Extra detail comes from post-coordination, clustering a stem code with extension codes. The system is digital-first, with a multilingual REST and FHIR interface, a built-in coding tool and the option to run it locally in Docker. WHO spreadsheets map ICD-10 codes across, but talks begun in 2017 on a two-way map with SNOMED CT had no timetable as of 2025.
Uptake is gradual. Farwaniya Hospital in Kuwait was the first general public hospital anywhere to switch. On 14 February 2023 the WHO counted 64 countries at some stage of implementation; by May 2024, 50 were running or widening pilots and 14 were using it in earnest. A JAMA article in July 2023 estimated that the United States would need at least 4 to 5 years to adopt it.
Source: ICD-11
