Multiple births after artificial induction of ovulation (AIO) are usually considered to be due to fertilisation of multiple ova. In the East Flanders Prospective Twin Study between 1978 and 1985 the frequency of zygotic splitting after AIO (1.2%) was significantly higher than the expected frequency (0.45%) among spontaneous twins and triplets. Moreover, after AIO the frequency of zygotic division was significantly higher in triplets than in twins. AIO seems to be the first identified biological mechanism influencing the monozygotic twinning rate.