During a 30 month investigation 3633 women were studied when the booked for antenatal care. Complement fixing antibodies against cytomegalovirus (CMV) were detected in 2078 (57 per cent) of the women and these sera were further tested for the presence of specific IgM antibodies by means of a solid-phase radioimmunoassay. Since specific IgM antibodies have previously been shown to persist for up to four months after primary CMV infection, their presence in a booking serum sample (mean gestation 15 weeks) was taken as presumptive evidence of a first trimester primary CMV infection. From theoretical considerations, 11.6 positive CMV-IgM reactions were predicted in these women and 11 were observed. A highly significant (p less than 0.001) excess of fetal death was seen in the infected women since three pregnancies ended in missed abortion (15 weeks), intrauterine death (29 weeks) and spontaneous abortion (24 weeks) whilst the remaining 8 women gave birth to apparently normal babies. Cord sera were available from 5 of the surviving babies and only one contained specific IgM antibodies, showing that transplacental spread of the maternal infections had not occurred in 4 of the 5 survivors.