In view of the evaluation of the National Immunisation Programme in the Netherlands the main objectives were to obtain insight into the immunity to diphtheria, tetanus and poliomyelitis, into the occurrence of pertussis and to improve serodiagnosis of pertussis.In a population-based nationwide sampling, 8359 sera (response 55%) were collected, and to gain access to orthodox reformed individuals refusing vaccination, in a sample from municipalities with low vaccine coverage 1589 sera (response 52.5%). In the nationwide sample, the prevalence of diphtheria and tetanus antibodies (≥0.01 IU/ml in toxin inhibition assay) was 88% and 84%, resp. In at least 90% antibodies (titre≥1:8 in neutralisation assay) against poliovirus types 1, 2 and 3 were measured. For those born after mass vaccination was introduced (<45 years) the prevalence of antibodies to diphtheria, tetanus, poliovirus types 1 and 2 was at least 92.5% and for poliovirus type 3 at least 80%. Diphtheria and tetanus antibodies decreased with age for those born before vaccination was introduced (≥45 years). Only 40% of orthodox reformed individuals had diphtheria and 60% had tetanus antibodies. Less than 70% had poliovirus type 1, 2 and/or 3 antibodies. We concluded that the Dutch immunisation programme induced long-term diphtheria, tetanus and poliomyelitis immunity. While adults are very well protected against poliomyelitis, a great number of adults lack diphtheria or tetanus antitoxin antibodies. These adults might benefit from diphtheria (re)vaccination; however, offering a primary tetanus vaccination to cohorts born before the introduction of vaccination would probably be more effective than routine revaccination. Introduction of C. diphtheriae or poliovirus in socio-geographically clustered orthodox reformed groups might constitute a danger of spread of these pathogens.Pertussis surveillance data from notifications, positive serology and hospital admissions (1976-98) showed a sudden increase in the number of pertussis cases in 1996-97. According to ...