Between 1987 and 1989, the prevalence of antibody to cytomegalovirus (CMV) was determined, by the ELISA method, in serum samples from 1494 apparently healthy subjects, 3–18 years old. Subjects were selected by a systematic cluster sampling from five geographical areas in Italy. The overall prevalence of antibody was 64·2%, increasing from 54·4% in 4–6-year-olds to 73·3% in subjects 17–18 years old (P < 0·01). Prevalence of antibody was significantly higher in females (P < 0·05) and in subjects residing in the South of Italy (P < 0·01). A significant association was found with sociodemographic factors. Subjects belonging to a household with six or more persons had a 1·5-fold risk (C.I. 95% = 1·11–2·04) and subjects whose fathers had less than 6 years of schooling had a 1·4-fold risk (C.I. 95% = 1·1–1·87) of previous exposure to CMV infection. The high prevalence (74·4%) of young women who are naturally immune when entering childbearing years does not guarantee that there will be a low risk of fetal infection.