The Centers for Disease Control and Prevention last estimated a national ectopic pregnancy rate in 1992, when it was 1.97% of all reported pregnancies. Since then rates have been reported among privately insured women and regional health care provider populations, ranging from 1.6-2.45%. This study assessed the rate of ectopic pregnancy among Medicaid beneficiaries (New York, California, and Illinois, 2000-03), a previously unstudied population.We identified Medicaid administrative claims records for inpatient and outpatient encounters with a principal International Classification of Diseases 9th Revision diagnosis code for ectopic pregnancy. We calculated the ectopic pregnancy rate among female beneficiaries aged 15-44 as the number of ectopic pregnancies divided by the number of total pregnancies, which included spontaneous abortions, induced abortions, ectopic pregnancies, and all births. We used Poisson regression to assess the risk of ectopic pregnancy by age and race.Four-year Medicaid ectopic pregnancy rates were 2.38% of pregnancies in New York, 2.07% in California, and 2.43% in Illinois. Risk was higher among black women compared with whites in all states (relative risk, 1.26; 95% confidence interval, 1.25-1.28; P < .0001), and among older women compared with younger women (trend for age, P < .001).Medicaid beneficiaries in these 3 states experienced higher rates of ectopic pregnancy than reported for privately insured women nationwide in the same years. Relying on private insurance databases may underestimate ectopic pregnancy's burden in the United States population. Furthermore, within this low-income population racial disparities exist.