An analysis was made of the demographic characteristics of 1000 women who were screened for serological evidence of cytomegalovirus (CMV) infection while receiving antenatal care in central London. The prevalence of antibodies against CMV was shown, by multiple discriminant analysis, to be significantly associated with non-Caucasian race (p less than 0.001), increasing maternal age (p less than 0.001) and poor social class (p less than 0.05). These results are interpreted as reflecting increased childhood exposure to CMV as a result of poor social environments. When data from 48 women who acquired primary CMV during pregnancy were analysed, infection was also related to non-Caucasian race (p less than 0.01), but in contrast, there was no demonstrable effect of social class, maternal age or marital status. We conclude that pregnant women acquiring the form of CMV infection with the greatest pathological potential for the fetus (primary infection) belong primarily to the middle-class sections of communities. Since middle-class women traditionally avail themselves of prophylactic measures, this result provides some optimism for the ultimate control of this common disease by immunization.