Previous surveys of the prevalence and natural history of ovarian cysts have been based on those ascertained by clinical examination. This study examines the fate of 137 adnexal cysts discovered during the course of 26 110 ultrasound scans in early pregnancy. Of these 10% were operated on in the first instance, and a further 2% required operation during pregnancy for painful complications. In 3% cysts were removed at caesarean section or in the puerperium. Half the removed cysts were neoplastic including one low grade carcinoma. Of the 120 cysts treated conservatively, 89% could not be detected on ultrasound examination later in pregnancy. All cysts greater than 8 cm should be removed, as should smaller cysts that are multilocular, thick-walled or semi-solid.