In Germany the annual number of systemic Haemophilus influenzae cases in unvaccinated children aged 3-60 months has recently been exceeded by the number of cases in children vaccinated at least once with the PRP-D, HbOC or OMP vaccines, which until 1995 have almost exclusively been used for H. influenzae b (Hib) vaccination. Most of the vaccinated children however could already have had more vaccinations at onset of disease. How much does an age-related suboptimal vaccination status increase the risk for systemic H. influenzae infections? A case control study was performed in West Germany. Cases with systemic H. influenzae infections were ascertained between 7/92 and 8/ 94 with an ongoing active hospital surveillance programme. Six age-matched population controls per case were recruited at random. Only vaccinated cases and controls were included in the study. The main exposure analysed in this study was suboptimal vaccination at censoring; for censoring ages (age at disease onset in cases and corresponding age in matched controls) > 6 months: one vaccination in 1st year only; > 18 months: two (three for combined vaccines with Hib + DT or DPT in one syringe) vaccinations in the 1st year of life but no booster vaccination. Suboptimal vaccination for age increased the risk for systemic H. influenzae infections by a factor of 4.74 (95%-CI 2.17-10.34). Following adjustment for confounders the odds ratio was 4.39 (95%-CI 1.74-11.07). Subgroup analyses showed that this risk was not related to the type of vaccine used. The risk for "no booster vaccination" in children aged > 18 months appeared even greater than the risk associated with one vaccination in the 1st year only.On schedule and complete Hib vaccinations are essential for an optimal effectiveness of Hib vaccination programmes. Booster vaccinations between 12 and 18 months are important if the PRP-D, HbOC and OMP vaccines are used for primary vaccination.