Following the recommendation by the National Health Council to include immunisation against Hib in the National Immunisation Programme in order to prevent mortality, severe morbidity and permanent disability as a consequence of invasive Hib infection, all children born after 1 April 1993 are offered immunisation. To assess the efficacy of this immunisation, surveillance has to be carried out. Apart from data on meningitis collected by the Netherlands Reference Laboratory for Bacterial Meningitis (RBM), data are needed on the incidence of other invasive Hib infections, as well as additional information on the cases, like vaccination status. Therefore since October 1993 active paediatric surveillance on invasive Hib infections is carried out by the RIVM via the Dutch Paediatric Surveillance Unit (NSCK). In this annual report on the Hib surveillance in 1994, the first effects of immunisation are described and a comparison is made of data collected through surveillance by paediatricians, public health laboratories and the RBM. A total of 129 paediatric reports of invasive infection by H. influenzae concerned 50 cases of just meningitis, 31 of meningitis with sepsis, 1 of meningitis with arthritis, 1 of meningitis with arthritis and osteomyelitis, 32 of epiglottitis including two cases with sepsis, 9 of only sepsis, 4 of only arthritis and 1 of only osteomyelitis. All proven infections occurred in children who had not or incompletely been vaccinated. One child with sepsis had had 3 vaccinations and became ill 7 months later ; the isolated Hi strain was not serotyped. Typing was performed in 85% of all isolates, of which 97% were of type b. Appropriate culturing was often omitted in cases of epiglottitis. The RBM seems to provide the most complete registration of meningitis/sepsis by Hib, the additional value of paediatric surveillance consists mainly in its ability to collect additional information on the cases and data on invasive infections other than meningitis/sepsis. The effect of immunisation against Hib became apparent in the number of cases of invasive infections which was lower than the estimated number before vaccination was introduced. Furthermore the peak incidence for meningitis no longer appeared in children aged 0, but in children aged one year. Adequate culturing and typing of specimen from a normally sterile site is essential ; since 1994 all such isolates can be send to the RBM for typing. As from 1 January 1995 the case definition for the paediatric surveillance is no longer restricted to type b infections ; this offers the opportunity to study changes in prevailing serotypes of Hi as well. Furthermore cellulitis is explicitly included in the case definition. Continuation of surveillance, at least until immunisation has covered the total risk group (children aged 0-5 years), is indicated to confirm the first effects of the inclusion of Hib vaccination in the NIP. The complementary systems of surveillance are appropriate to doing this.