One hundred and thirty-three pregnant women who delivered at St Thomas' Hospital, in 1990 were noted to require rubella vaccination post partum. Fifty-three (39%) had completed a telephone questionnaire in order to determine reasons for susceptibility to rubella. Laboratory reports confirmed that 92 women were rubella seronegative and 27 had low levels of antibody. Of the 53 women interviewed, 25 gave a history of one or more rubella immunizations, 20 had no history of immunization and vaccination history was unknown for eight. Eleven of the 20 unvaccinated women had not been at school in the UK between 11 and 14 years of age. Eighty-seven per cent of the patients' general practitioners had no knowledge of their patients' rubella antibody status. Ninety-four per cent of the 133 women received rubella vaccine post partum. The Department of Health guidelines should be more vigorously implemented in order to identify and immunize remaining rubella susceptible women of child-bearing age. Susceptibility among women with a history of rubella immunization suggests that the seroconversion rate following rubella immunization in clinical practice may be lower than in vaccine trials.