A large (165 × 235 × 250 mm) solitary, unilocular cyst with a thin, smooth wall and homogeneous anechoic content was detected during a routine ultrasound scan at 24 weeks of gestation in an asymptomatic 39-year-old woman with a singleton pregnancy. The cyst was aseptate, lacked mural blood flow and was not associated with ascites. It was located in the central abdominal area above and anterior to the uterus. Gradually increasing abdominal discomfort developed, and a laparotomy was performed at 27 weeks; the cyst was removed after aspiration of 6.3 L of serous fluid and the ipsilateral ovary was preserved. Pathological examination indicated a large luteinized follicular cyst of pregnancy. A healthy male infant was delivered vaginally at term. A rapidly enlarging ovarian mass in pregnancy poses significant diagnostic problems. Large luteinized cysts of pregnancy are uncommon and thought to involve stimulation by human chorionic gonadotropin (hCG), or increased tissue sensitivity to hCG. A literature search identified four previous cases that had been detected prenatally. With one exception, the cysts appeared to enlarge during pregnancy, eventually becoming symptomatic, and two previous cases also required removal of the cyst before birth. Adverse pregnancy outcome was only reported in one of the previous cases. In summary, large luteinized cysts of pregnancy are an uncommon type of cystic mass particular to pregnancy, characterized by the combination of a benign appearance and a tendency to enlarge rapidly, eventually becoming symptomatic and most often necessitating surgery.