We report the first case of an intrauterine treatment for cystic hygroma. Guided by ultrasonography, we first removed intracystic fluid from two cysts and then injected OK-432 into each fetal cyst at 21 and 28 weeks of gestation. No re-enlargement of the cysts was subsequently observed. At 38 weeks of gestation, a male infant was delivered transvaginally. Only a slight skin fold was observed in the nuchal area of the neonate, indicating the effectiveness of OK-432 for the intrauterine treatment of cystic hygroma.