A literature review of the occurrence of multiple pregnancies associated with artificial induction of ovulation is reported. This report considers three treatment schedules: (1) clomiphene citrate; (2) human pituitary gonadotrophin with human chorionic gonadotrophin; and (3) human menopausal gonadotrophin with human chorionic gonadotrophin. The majority of the increase in twinning is related to hyperstimulation of the ovary by these medications, resulting in dizygotic twinning. The true incidence of twin pregnancy cannot be calculated because the vital statistics of all nations report live birth rates. Increased rates of fetal wastage, late abortion and prematurity associated with the occurrence of multiple pregnancies are overlooked by these statistics. The increased incidence of twinning appears to be related to the type and dosage of medication used, and the patient's underlying problem.