Medical procedures that use ionizing radiation have grown rapidly in volume over the last two decades and constitute a substantial portion of the collective radiation doses to the U.S. population. The purpose of this study is to describe the components of this growth. Summarized claims data from Medicare are used to describe trends for the period 1986-2005; supplemental data from other payers and surveys are used for verification and to describe age distributions of those who have these procedures. A notable trend is the rapid growth of CT and nuclear medicine, with CT volume per fee-for-service Medicare enrollee growing, on average, at 8% per y and nuclear medicine at 7% per y during the period 1986-2005. Cardiac procedures-nuclear medicine and interventional radiology-grew at over 15% per y per fee-for-service enrollee during the same period. The share of nuclear medicine procedures performed in physician offices increased from 10% in 1986 to 55% in 2005; the share of CT in the emergency room increased from 3% in 1992 to 17% in 2005. With this expansion in imaging volumes across practice settings, there is an increased need for radiation safety education of ordering physicians, imaging physicians, and patients, so that adequate consideration is given to radiation risk when determining the appropriateness of a prescribed procedure.