Web-based annotation of three dimensional medical imagery is provided. In various embodiments, a plurality of two dimensional medical images is read from a data store. The plurality of two dimensional images is a subset of a three dimensional medical imaging study. The plurality of two dimensional medical images is provided to a remote user. Regional annotations for each of the plurality of medical images are received from the remote user. A volumetric description of the three dimensional imaging study is generated by interpolation of the regional annotations. The volumetric description of the three dimensional imaging is provided for rendering and display to the remote user.