Several investigators have reported a 40% increase in the prevalence of twinning among women who have taken folic acid or multivitamins containing folic acid at the time of conception. Given that infant morbidity and mortality are greatly increased among twins, such a large increase in twinning could have serious implications. We undertook this study to determine if US fortification of enriched cereal-grain products with folic acid was associated with an unexpected increase in the prevalence of twinning in the state of Texas. We examined 1 003 207 deliveries conceived in Texas, between 1 January 1996 and 31 December 1998. We compared the prevalence of twin deliveries conceived before, during and after fortification with folic acid, mandated to begin on 1 January 1998. Comparing pregnancies conceived in 1997 with those conceived in 1996, we observed a 2.4% yearly increase in twinning, 1.024 [0.98, 1.07]. Comparing pregnancies conceived in 1998 with those conceived in 1997, we observed a 4.6% yearly increase in twinning, 1.046 [1.00, 1.09]. These increases were adjusted for maternal age, race, education, parity and season of conception. The size and pattern of these increases are consistent with the ongoing increase in twinning of 1-4% per year which began in the US prior to fortification.