The epidemiology of pertussis may be approximately represented by an equation that relates the rate of increase in incidence of pertussis to the contagiousness of pertussis, the number of contacts per case, and the proportion of the population susceptible. By use of this equation it is possible to estimate the proportion of the population that is susceptible to pertussis at any stage of a pertussis epidemic. Data from pertussis epidemics in England and Wales in the last 10 years suggest that the level of immunity to pertussis has fluctuated between 92% and 94% of the total population. During this period pertussis vaccine uptake in England and Wales has risen from a low of 30% to the present level of approximately 80%. Comparison with other countries suggests that a further increase in vaccine uptake to levels above 90% would reduce pertussis incidence to less than one-tenth of its present level. These aspects of pertussis epidemiology are likely to be of relevance in the planning and interpretation of future pertussis vaccine trials, such as are expected to start in England shortly.