Sera from 2,000 women in the early stages of pregnancy in 1980-1981 were tested for antibodies to Epstein-Barr virus. Of the 2,000 women, 1,386 attended an obstetrical clinic (average age, 21 years) and 614 attended a private practice (average age, 28 years). Only eight (0.6%) of the women attending the obstetrical clinic were seronegative (less than or equal to 20 years, five [0.8%] of 650; 21-30 years, three [0.5%] of 648; and greater than or equal to 31 years, zero of 88), whereas 18 (2.9%) of the women attending the private practice were seronegative (less than or equal to 20 years, two [4.8%] of 42; 21-30 years, 12 [3.0%] of 388; and greater than or equal to 31 years, four [2.3%] of 184). Probable recent seroconversions were detected in 35 women. Sera from 185 women pregnant in 1959-1965 were tested for antibodies to Epstein-Barr virus; 17 (9%) were seronegative, compared with 26 (1.3%) of the women in 1980-1981 (P less than 0.01).