The Radiological Physics Center (RPC) was established upon the recommendation of the Committee on Radiation Therapy Studies (CRTS) under the sponsorship of the American Association of Physicists in Medicine (AAPM) to insure the correctness and consistency of radiation dosimetry between institutions involved in clinical trials. Review of dosimetry is accomplished by visits to the institutions by a physicist, irradiation of anatomical phantoms containing thermoluminescent dosimeters, and by review of dosimetry on patients entered into clinical trials. When errors are discovered, the RPC cooperates with the institution to rectify the errors. To date there are 20 active group trials served by the RPC and 3 intergroup trials which are underway and will be served by the RPC and about 4 trials in the planning stage. Visits have been made to 105 institutions. Dosimetry discrepancies have been found at 28 institutions of which 23 have been resolved. Efforts are made to develop calculative methods and efficient techniques for measurements that are important to clinical trials. This information and other knowledge gained are communicated to the radiological community.