Background: A significant increase in the frequency of twins has been described in the literature as a secondary effect of perinatal supplementation with folic acid. Aim: To verify if the frequency of twins increased in a Chilean hospital, after the start of flour folic acid supplementation. Material and methods: The frequency of twins was compared in the University of Chile Clinical Hospital, before and after the start of flour fortification with folic acid. Results. There was a 34% increase in. the frequency of twins during the post fortification period (2001-2004) as compared with the previous period (1998-2000). A decrease in the frequency of male twins, when compared with single newborns, was also observed in the post fortfication period. There was a higher frequency of stillborns among twins, when compared with single newborns. Conclusions: The frequency of twin pregnancies increased after flour folic acid supplementation in sample of Chilean population.