Objective: Cytomegalovirus immunoglobulin was administered to a pregnant woman with primary cytomegalovirus infection and placental involvement of 1 twin fetus, in whom growth restriction had developed. Study Design: Inhibition of viral activity was attempted by administration of high-titer cytomegalovirus neutralizing antibodies for therapy of the involved fetoplacental unit and prevention of cytomegalovirus infection in the uninfected twin fetus. Results: After cytomegalovirus immunoglobulin infusions the placental edema decreased and the infected fetus started to grow once again, showing at birth only hepatosplenomegaly associated with viruria and cytomegalovirus deoxyribonucleic acidemia. Moreover, cytomegalovirus immunoglobulin G avidity increased and cell-mediated immunity improved. The other twin, who had negative results of cytomegalovirus culture and deoxyribonucleic acid detection at birth, was found to have cytomegalovirus deoxyribonucleic acid in the urine after 1 week. From the age of 9 months, however, both twins had persistent negative results of cytomegalovirus deoxyribonucleic acid detection. Conclusion: Although large-scale studies are needed to establish the real efficacy and the best therapeutic regimen, cytomegalovirus immunoglobulin may be considered for treatment or prevention of fetal cytomegalovirus infection. (Am J Obstet Gynecol 1999;180:1222-6.)