An overview is presented of serological and virological studies on poliovirus immunization and circulation in the Netherlands, performed between 1979 and 1991. In this period, only three patients with poliomyelitis were notified. All had acquired the infection abroad. The vaccinations in the national immunization programme, using inactivated poliovirus vaccine, build a strong immunity. This can also be seen in age-stratified serological profiles of the Dutch population. In these surveys, persons from the time at which vaccination was offered have neutralizing antibodies. Older persons, especially those born between 1930 and 1945, sometimes lack antibodies. However, 85-90% of them show a rapid booster response upon vaccination, demonstrating immunological memory. Hence, they will be protected against poliomyelitis upon contact with wild poliovirus. Virological data show a regular import of poliovirus, especially in adoptive children tested on entry into the Netherlands, coming from developing countries. Nearly all other virus isolates in Dutchmen were related to import from such countries. None of the imported patients or other persons in whom poliovirus was detected spread the virus over the country. It demonstrates that as a rule the herd immunity of the well-vaccinated Dutch population is good. Exceptions occur, however, as demonstrated by the epidemics in 1978 and 1992. Large socio-geographic clusters of susceptible people who refuse vaccinations are not sufficiently protected.