Dizygotic twinning is familial, suggesting that there may be an inherited abnormality of the control of ovulation that predisposes to double ovulation and, therefore, dizygotic twins. The present study examines 17 mothers of dizygotic twins (MODZT) and 8 control mothers of singletons by daily blood sampling throughout an entire menstrual cycle. Blood samples were assayed for LH, FSH, estradiol, progesterone, and inhibin. The process of follicular development was followed by transvaginal ultrasound. The pituitary LH response to iv GnRH was also assessed. Three of the 16 MODZT double ovulated during the study compared to none of the 8 control mothers (P < 0.05). The number of small follicles (<6 mm) declined significantly in control women at midcycle, but not in MODZT. There was no significant difference in serum FSH, LH, estradiol, or inhibin levels between the 2 groups at any stage of the menstrual cycle. During the follicular phase, serum progesterone levels were significantly higher in MODZT. The response to GnRH stimulation was not different between MODZT and controls. In conclusion, this study demonstrates an increased tendency to double ovulate in MODZT that may be due to a reduced rate of atresia in advanced follicles. Furthermore, the elevated progesterone levels in MODZT during the follicular phase suggest altered intrafollicular steroidogenesis that is independent of gonadotropins.