A systematic prospective study of the incidence of cytomegalovirus (CMV) infection during pregnancy and contamination of the neonate was set up in a Maternity Hospital in Lyons. Over 7,000 pregnancies have been followed up since 1982 with serological and virological investigations. CMV infection was found in 5 p. 100 of patients. A urine collection was obtained at birth in 641 of the neonates. CMV was isolated in 16 cases and the transmission of the infection was studied with respect to the immune status of the mother. When CMV infection was demonstrated in the mother by serology and excretion of the virus, 45.5 p. 100 of babies were contaminated (10/22). In cases of reinfection, the incidence of neonatal contamination was 3.1 p. 100 (3/98). Ten of the 11 primary infections diagnosed were associated with viral excretion and the infection was transmitted to the child in 50 p. 100 of cases (5/10). The high incidence of transmission of CMV infection after a primary maternal infection supports data previously published by Stagno and Kumar in the U.S.A. and Ahlfors in Sweden.