Background: Little population-based information is available on the use of chiropractic services. Methods: We analyzed data from the RAND Health Insurance Experiment (HIE), a community-based study of the use of health services. Insurance claim forms for all fee-for-service patients who completed the study were examined for all visits coded as being seen by a chiropractor. Services provided and patient-specified symptoms were taken from these forms. Population-based use rates were calculated for each HIE site. Use rates and services were calculated separately for first visits and repeat visits. Results: There were 5,279 persons who contributed 19,021 person-years of exposure during the study; 395 different persons used 7,873 chiropractic services for a visit rate of 41 per 100 person-years and rate of use of 7.5 percent. Forty-two percent of all visits were for pain in the back. Spinal manipulation accounted for 61 percent of all services provided. Compared to non-users, users tended to be White, middle-aged, married, and high school educated. Seven-fold geographic variations in the use of chiropractic services were seen. Conclusions: Chiropractors deliver a substantial amount of health care to the US population, and there are significant geographic variations in the rate and intensity of use of chiropractic services.