Intrauterine adenovirus infection has been associated with nonimmune hydrops. We describe a twin pregnancy in which adenovirus was demonstrated by polymerase chain reaction in the amniotic fluid of one twin afflicted by hydrops. Adenovirus was absent in the amniotic fluid of the other twin. A 25-year-old, gravida 3, para 1, with twin gestation at 20 weeks' gestation was noted on ultrasound anomaly screen to have hydrops associated with twin B. Polymerase chain reaction analysis of amniotic fluid revealed adenovirus in twin B with none noted in twin A. At 26 weeks' gestation both infants died. This case demonstrates differential transmission of adenovirus in a twin pregnancy. One twin developed hydrops fetalis possibly secondary to adenovirus infection.