We analysed time trends in the incidence of invasive Haemophilus influenzae disease in Switzerland between 1980 and 1993 to investigate whether conjugated H. influenzae type b vaccines, licensed in Switzerland in May 1990, confer indirect protection to children in older, non-vaccinated age groups. Data were obtained from the records of 39 Swiss paediatric clinics for 2,857 children 0-16 years old with invasive H. influenzae disease. Incidence time trends were analysed by Poisson regression. The diseases incidence decreased by 80% among 0-4-year-old children (i.e. those eligible for vaccination) between 1990 and 1993. Among children aged 5-16 years, there also was an abrupt 50% fall in the incidence of H. influenzae meningitis after 1990. However, the incidence of epiglottitis in this age group had started to decline before conjugated vaccines became available, with no additional decline thereafter. While our results suggest some indirect protection conferred to older, non-vaccinated children through the administration of conjugated vaccines to younger children, they also argue that underlying time trends of invasive H. influenzae disease need not be considered when interpreting incidence rates in the vaccine era.