Hepatitis A is an infection transmitted by the fecal-oral route. Endemicity within a specific country is directly related to sanitation and hygienic standards, while being inversely related to socioeconomic conditions. We studied how the process of urbanization witnessed in Madrid had influenced the transmission of hepatitis A infection. In the Madrid Autonomous Region, this process first began in the early sixties and was not brought to a close until the late seventies. Catalytic models were used to estimate the annual infection rate, lambda, on the basis of seroprevalence data stratified by age. A cohort effect related to a fall-off in infancy-related hepatitis A virus (HAV) is to be observed in the results for the last few years. The model permits four birth cohort-based groups to be differentiated by lambda: individuals born pre-1960, lambda = 0.082 (95% CI 0.095-0.070); those born in the early sixties, lambda = 0.052 (95% CI 0.060-0.042); whose members were born in the late sixties, lambda = 0.033 (95% CI 0.041-0.025); and those born in the late seventies, lambda = 0.017 (95% CI 0.020-0.013). The first group includes those born before the urbanization process had started. The second and third groups coincide with the development stage of that process, hence exhibiting transitional rates. The fourth group reflects the process in its consolidation stage. This reduction in the transmission of infection has changed the manner of presentation, so that while isolated cases or small outbreaks tend to be more common nowadays, occasionally epidemics may evolve explosively. The average age at presentation has risen and the likelihood of symptomatic infection is higher.