The World Health Organization (WHO) is well under way with the new revision of the International Classification of Diseases (ICD-11). The current revision process is significantly different from past ones: the ICD-11 authoring is now open to a large international community of medical experts, who perform the authoring in a web-based collaborative platform. The classification is also embracing a more formal representation that is suitable for electronic health records. We present the ICD Collaborative Authoring Tool (iCAT), a customization of the WebProtégé editor that supports the community based authoring of ICD-11 on the Web and provides features such as discussion threads integrated in the authoring process, change tracking, content reviewing, and so on. The WHO editors evaluated the initial version of iCAT and found the tool intuitive and easy to learn. They also identified improvement potentials and new requirements for large-scale collaboration support. A demo version of the tool is available at: http://icatdemo.stanford.edu.