OBJECTIVES: This article examines the effect of abortion legalization on fertility rates in the United States. METHODS: Fertility rates were compared over time between states that varied in the timing of abortion legalization. RESULTS: States legalizing abortion experienced a 4% decline in fertility relative to states where the legal status of abortion was unchanged. The relative reductions in births to teens, women more than 35 years of age, non-White women, and unmarried women were considerably larger. If women did not travel between states to obtain an abortion, the estimated impact of abortion legalization on birth rates would be about 11%. CONCLUSIONS: A complete recriminalization of abortion nationwide could result in 440,000 additional births per year. A reversal of the Roe v Wade decision leaving abortion legal in some states would substantially limit this impact because of the extent of travel between states.