Haemophilus influenzae causes various infectious diseases such as meningitis, septic arthritis, bacteriemia, ampyeme and cellulites. Although all subtypes of H. influenzae cause these infectious conditions, H. influenzae type b (Hib) is the most common subtype with a incidence of 95 % and it is known to be a major cause of mortality and morbidity among children under 5 years of age in Asia. Hib vaccine is not entered in routine immunization program in children in Iran yet. The present results show that most Iranian infants that are not vaccinated with Hib-conjugate vaccines are susceptible to Hib diseases, which confirm the previous findings. We found that the protective titer of antibody against Hib is very low in children under 1 year old and gradually increases with age as it shown in other study. The decrease in the prevalence of Hib-associated disease in older children appears to be associated with age-related increases in anti- capsular polysaccharide (anti-CP) titers.