Since 1992-93 vaccination against Haemophilus influenzae type b (Hib) has been included in the general Swedish childhood vaccination programme. The aim of the present study is to describe the epidemiology, identify and describe vaccine failures and calculate vaccine effectiveness during the first 6 y after introduction of vaccination against Hib. Laboratory reports of blood and cerebrospinal isolates to the Swedish Institute for Infectious Disease Control were used as the source for identifying the patients. Additional information was subsequently obtained from physicians and parents of children who had developed the disease during the study period. Vaccine failures were identified and vaccine effectiveness calculated. During the study period, 152 cases of invasive H. influenzae were identified in the age group 0-14 y. During the 6-y period, 6 true vaccine failures, 6 apparent vaccine failures and 1 possible vaccine failure were found in nearly two million vaccinated child-years. The effectiveness of the Hib vaccination in the birth cohort of children 1993 to 1997 in Sweden was calculated to be 96.1% (95% confidence interval 94.2-97.5). The study supports earlier studies from several countries that conjugated Hib vaccination introduced in general childhood vaccination programs is effective and substantially decreases suffering from invasive Hib diseases.