Objective: To measure the effect of competition among fertility centers on pre gnancy and high-order multiple (HOM) gestation rates after IVF. Design: Retrosp ective cohort study. Setting: Four hundred eight fertility clinics registered wi th the Society for Assisted Reproductive Technology as providing IVF services in 2000. Competition was defined as number of clinics in a geographically defined area. Demand for services was based on the population of reproductive-aged wome n. Patient(s): Three hundred eighty-one fertility clinics reporting clinical ou tcomes. Intervention(s): Pregnancy rates, HOM gestation rates, population of rep roductive-aged women, and number of competing clinics were calculated for each clinic from Society for Assisted Reproductive Technology and census data. Main O utcome Measure(s): The clinic HOM gestation rate (percentage of pregnancies that were HOM) and age-adjusted pregnancy rate. Result(s): The number of clinics in an area of competition ranged from 1 to 22. The HOM gestation rate per clinic r anged from 0%to 50%. As demand increased, competition increased. As competitio n increased, the number of HOM pregnancies per clinic decreased. In areas of low competition (1 to 2 clinics) the clinic HOM gestation rate was 8.43%, in areas of intermediate competition (3-7 clinics) 8.39%, and in areas of high competi tion (8-22 clinics) 8.24%. In areas with intermediate demand, high levels of c ompetition resulted in fewer HOM pregnanc ies than intermediate competition (relative risk 0.56, 95%confidence interval 0 .36-0.89) or low levels of competition (RR 0.57, 95%confidence interval 0.35- 0.94). Age-adjusted pregnancy rates did not differ by level of competition. Con clusion(s): According to these data, the risk of HOM gestation decreases with in creasing competition among clinics; however, pregnancy rates are unaffected. Objective: To measure the effect of competition among fertility centers on pre gnancy and high-order multiple (HOM) gestation rates after IVF. Design: Retrosp ective cohort study. Setting: Four hundred eight fertility clinics registered wi th the Society for Assisted Reproductive Technology as for providing IVF services in 2000. Competition was defined as number of clinics in a geographically defined area. Demand for services was based on the population of reproductive-aged wome n. Patient (s): Three hundred eighty-one fertility clinics reporting clinical ou Interventions (s): Pregnancy rates, HOM gestation rates, population of repoductive-aged women, and number of competing clinics were calculated for each clinic from Society for Assisted Reproductive Technology and census data. Main O utcome Measure (s): The clinic HOM gestation rate (percentage of pregnancies that were HOM) and age-adjusted pregnancy rate. Result (s): The number of clinics in an area of ​​competition ranged from 1 to 22. The HOM gestation rate per clinic r anged from 0% to 50%. As demand increased, competition increased. clinic HOM gestation rate was 8.43%, 8.39% in areas of intermediate competition (3-7 clinics), and 8.24% in areas with high demand (8-22 clinics). in fewer HOM pregnanc ies than intermediate competition (relative risk 0.56, 95% confidence interval 0.36-0.89) or low levels of competition (RR 0.57, 95% confidence interval 0.35- 0.94). Age-adjusted pregnancy rates did not differ by level of competition. Con clusion (s): According to these data, the risk of HOM gestation decreases with in creasing competition among clinics; however, pregnancy rates are unaffected.