A prospective study was conducted over a 6 year period to determine the natural history of uterine leiomyomas in pregnancy. All pregnant women with myomas detected during obstetric ultrasonographic examination and with three or more sonographic examinations were included; 134 patients fulfilled the inclusion criteria. Sonograms were obtained at 2 week intervals until 20 weeks of gestation, and monthly thereafter. The following observations were made: (1) the majority of myomas 5 cm or less in average diameter could no longer be seen during pregnancy; (2) the majority of myomas greater than 5 cm in diameter tended to remain stable or decrease in size during pregnancy; and (3) multiple myomas were less likely to disappear than solitary myomas.