The authors assessed concordance of local Medicare health care utilization data on cataract surgery and estimates generated using the databases of the Rochester Epidemiology Project, which capture virtually all medical care received by residents of Olmsted County, Minnesota. The Rochester Project databases identified 1,353 primary cataract extractions performed in Olmsted County between October 1989 and December 1993 among county residents aged > or = 65 years. Medicare data identified 1,148 claims-84.8% of the number of procedures identified by the Rochester Project. Ratios of numbers of encounters (Medicare/Rochester Project) were 189/350 (0.540) for 1992 versus 959/1,003 (0.956) for the other years combined. Changes in Medicare data file transfer procedures may have produced the 1992 data shortfall. Medicare data should periodically be compared with source data to assess concordance.