Absorbed-dose-to-water calibrations are important to the medical community to facilitate the accurate determination of doses delivered to tumors during external-beam cancer therapy. The first version of this document offered an absorbed-dose-to-water calibration service based on a graphite calorimeter as the primary standard. However, the use of this calorimeter necessitated calculations to convert the measurement from graphite to water. In 1989, a water calorimeter was introduced at the National Institute of Standards and Technology (NIST), which was to replace the graphite calorimeter as the primary standard. Though the calculations necessary for conversion factors were eliminated with this new technology, a calibration service based on the water calorimeter was not developed at this time. Despite the fact that the service was available, the medical physics community did not take advantage of it and used chambers calibrated in terms of exposure (in units of roentgen) to calibrate their radiotherapy 60Co and high-energy electron accelerator x-ray. A protocol, commonly known as TG21, developed by the American Association of Physicists in Medicine (AAPM), involves many calculations to arrive at the quantity desired by the medical physicist in the practicing clinic, cGy/MU (centiGray/monitor unit). The AAPM has initiated a new protocol through Task Group 51 which involves absorbed-dose-to-water calibrations of ion chambers commonly used in the calibration of the clinical radiotherapy photon and electron beams.NIST has developed and is now prepared to offer the absorbed-dose-towater calibration service for ionization chambers based on a water calorimeter standard developed by Steve Domen at NIST. This document outlines the steps that have been taken to develop this service including a brief description of the Domen water calorimeter. The procedures that are involved in the calibration of an ionization chamber for this quantity are presented along with results from recent comparisons of the NIST with the Bureau International des Poids et Mesures (BIPM) in France and the National Radiation Council Canada (NRCC).