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We report an enhanced prospective survey of invasive Haemophilus influenzaeinfections that has defined the pattern of invasive disease in five English regions for 2 years before and 4 years after the introduction of the H. influenzaetype b (Hib) vaccination program. During the prevaccination period the majority of cases of invasive H. influenzaewere caused by type b; most (89%) of these infections occurred in children <5 years of age and the most common presentation was meningitis. Since the introduction of routine immunization of infants with conjugate Hib vaccine, there has been a 16-fold reduction in the annual attack rate of invasive Hib disease recorded in children <5 years of age. This reduction is of a magnitude similar to that observed in other countries with Hib vaccination programs. The number of infections caused by non-type b H. influenzaehas shown a small but progressive increase over the same period, emphasizing the need for continued surveillance. There was no increase in the number of infections caused by other serotypes.