Background. Intrauterine transmission of cytomegalovirus (CMV) can occur whether a mother has prior immunity or acquires CMV for the first time during pregnancy. The degree of protection afforded an infected infant by the presence of antibody in the mother before conception is uncertain. Methods. We compared the outcomes of CMV-infected infants born to mothers who acquired primary CMV infection during pregnancy (primary-infection group) with those of CMV-infected infants born to mothers with immunity (recurrent-infection group). Screening for viruria identified 197 newborns with congenital CMV infection. Stored serum samples were used to categorize maternal infection as either primary or recurrent. We followed 125 infants from the primary-infection group and 64 from the recurrent-infection group. Serial medical, audiologic, psychometric, and eye examinations were used to identify sequelae of CMV infection. Results. Only infants in the primary-infection group had symptomatic CMV infection at birth (18 percent). After a mean follow-up of 4.7 years, one or more sequelae were seen in 25 percent of the primary-infection group and in 8 percent of the recurrent-infection group. Thirteen percent of infants whose mothers had primary infection during pregnancy had mental impairment (IQ ≥70), as compared with none of those whose mothers had recurrent CMV infections. Sensorineural hearing loss was found in 15 percent of those in the primary-infection group and in only 5 percent of those in the recurrent-infection group. Bilateral hearing loss was identified only among children in the primary-infection group (8 percent). Conclusions. The presence of maternal antibody to CMV before conception provides substantial protection against damaging congenital CMV infection in the newborn. Primary maternal infection during pregnancy is associated with more severe sequelae of congenital CMV infection. (N Engl J Med 1992;326:663–7.). © 1992, Massachusetts Medical Society. All rights reserved.