Using Medicare hospital claims records and beneficiary enrollment data, the investigators describe the epidemiology of inpatient pacemaker procedures (International Classification of Diseases, Ninth Revision, Clinical Modification codes 37.80 to 37.89) in Medicare enrollees. From 1990 to 2000, the age-standardized inpatient pacemaker procedure prevalence (per 100,000 enrollees) increased from 325.4 to 504.4 in all Medicare beneficiaries. The prevalence increased significantly with age; was less for women than for men; and was less for blacks, Hispanics, and Asians than for whites.