Background: Although most insurance companies in the United States do not cover in vitro fertilization, a few states mandate such coverage. Methods: We used 1998 data reported to the Centers for Disease Control and Prevention by 360 fertility clinics in the United States and 2000 U.S. Census data to determine utilization and outcomes of in vitro fertilization services according to the status of insurance coverage. Results: Of the states in which in vitro fertilization services were available, 3 states (31 clinics) required complete insurance coverage, 5 states (27 clinics) required partial coverage, and 37 states plus Puerto Rico and the District of Columbia (302 clinics) required no coverage. Clinics in states that required complete coverage performed more in vitro fertilization cycles than clinics in states that required partial or no coverage (3.35 vs. 1.46 and 1.21 transfers per 1000 women of reproductive age, respectively; P<0.001) and more transfers of frozen embryos (0.43 vs. 0.30 and 0.20 per 1000 women of reproductive age, respectively; P<0.001). The percentage of cycles that resulted in live births was higher in states that did not require any coverage than in states that required partial or complete coverage (25.7 percent vs. 22.2 percent and 22.7 percent, respectively; P<0.001), but the percentage of pregnancies with three or more fetuses was also higher (11.2 percent vs. 8.9 percent and 9.7 percent, respectively; P=0.007). The number of fresh embryos transferred per cycle was lower in states that required complete coverage than in states that required partial or no coverage (P=0.001 and P<0.001, respectively). Conclusions: State-mandated insurance coverage for in vitro fertilization services is associated with increased utilization of these services but with decreases in the number of embryos transferred per cycle, the percentage of cycles resulting in pregnancy, and the percentage of pregnancies with three or more fetuses.