We wished to establish the frequency, regularity and laterality of multiple ovulation in mothers of dizygotic (DZ) twins and controls. Subjects had regular menses and were not using oral contraceptives. Ovarian ultrasound scans were taken over a number of cycles in 21 mothers of DZ twins and 18 controls (including 13 mothers of monozygotic [MZ] twins). Multiple large follicles (greater than or equal to 12 mm diameter) were seen significantly more frequently in mothers of DZ twins (13/21 mothers, 24/77 cycles, average +/- SE follicles/cycle 1.34 +/- 0.11) than controls (2/18 mothers, 3/31 cycles, average 1.10 +/- 0.08). Both ipsilateral and contralateral multiple follicles were observed. In one case a mother of DZ twins showed multiple large follicles in 7 out of 10 cycles in which she was scanned, including both ipsi- and contralateral patterns of occurrence.