Pertussis remains present in the industrialized world, although the widespread use of whole-cell pertussis vaccine has led to a dramatic decline of the disease. Great differences in immunization policies and coverage explain the uneven epidemiological pattern across the developed world. Different situations can be described. In some countries such as Japan, UK and Italy, concerns about the occurrence of severe adverse events following pertussis vaccination in the 70's led to a reduced acceptance and thus, to a significant decrease in immunization coverage. In Sweden, the protective efficacy of pertussis vaccine was questioned and vaccination was discontinued in 1979. In all these countries, severe epidemics of pertussis were observed in the following years, and immunization was then reinforced in some (UK). In Japan, considerable efforts were made to develop new and safer pertussis acellular vaccines which have replaced the whole-cell vaccine since 1981. In other countries such as the USA and France, despite a continuous immunization programme maintaining a sustained and high coverage, a resurgence of the disease was observed in association with a modification of the age-distribution. Vaccinated adults became susceptible to the infection because of progressive waning, vaccine-induced immunity. These patients contaminated very young and thus, unvaccinated infants. These observations underline the need for early immunization in infants as well as the importance of late booster doses to be given to older children or adults.(ABSTRACT TRUNCATED AT 250 WORDS)