A seroepidemiological study was carried out in Switzerland to define the population susceptible to rubella among women of childbearing age. IgG antibodies to rubella virus were determined in 9,046 women giving birth between 1 August 1990 and 30 September 1991 in 23 of 26 Swiss cantons. These sera represented 10-20% of the yearly total number of births in each Swiss canton. Anti-rubella IgG was measured by an automated enzyme-linked fluorescent assay for use with a commercial system (Vidas Rub IgG, bio-Mérieux, France). Before the study population was screened, the commercial system was compared to the traditional hemagglutination-inhibition (HAI) test using 500 consecutive samples from parturient women. The sensitivity was 97.7%, the specificity was 100%, and agreement between the two tests was 97.8%. The discrepancies corresponded to very low titres of antibodies as measured by HAI. The seroprevalence of rubella nationwide in women of childbearing age in Switzerland was 94.3%. The seroprevalence was higher (96.5%) in the 5,677 women of Swiss nationality than in the 3,090 women of a different nationality (90.4%) (p < 0.001). In Swiss women the seroprevalence of rubella did not increase significantly with age and was identical in primiparous and in multiparous women, thus indicating that women of childbearing age are probably not sufficiently immunised.