The current study is an attempt to study the causes and some risk factors why preterm and low birth weight infants lag behind in their first immunization status in Hilla city the provincial capital of Babylon Governorate in central Iraq. It also attempts to assess the effects of different demographic variables related to both infants and their parents, on delayed up-to-date immunization. Data collection was carried out during the period from the beginning of April to the end of July 2008,with an average of 4-5 working hours per day, 5 days per week .A cross sectional study was conducted in nine primary health care centers in Hilla city. A total of 450 infants born preterm or who had a low birth weight and who were previously discharged of Hilla's two neonatal intensive care units were included. A direct interview of each infants' parent was done using a specially designed questionnaire form for this purpose. The parents were asked about the socio-demographic characteristics and the biological characteristics of their infants. The reasons for delayed immunization of the preterm and low birth weight infants, were recorded also. To verify the authenticity of the history and information collected from the parents, the information regarding both gestational age at birth and birth weight were double checked by reviewing the child's admission files at Hilla's two Neonatal Intensive Care Units. On average these infants were vaccinated at an age of 75 days being delayed for 68 days.The results showed that the delay is not due to a single factor , but is due to a combination of different factors. This study showed that the most common cause for delayed initiation of immunization of such children, is family fear to vaccinate thinking that their infants are too frail because of their low birth weight and prematurity .This reason accounts for well over one third of the reasons and in fact it accounts for 41.3% of all causes. Health centre personnel in Hilla city for some reason or another put some arbitrary weights at which they allow infants to be vaccinated . Refusal of local and major health centers to vaccinate accounts for 38% of reasons why preterm and low birth weight infants were not vaccinated on time. Not all pediatricians and general practitioners emphasize the importance of vaccination of these high risk infants. In fact 7.8% of reasons for delayed up-to-date immunization of such children is according the advice of private clinic doctors who advice against early vaccination.There was a significant relation between maternal and paternal education and employment and timing of vaccination. The higher the educational level of parents and those parents who were employed tend to delay immunization less. So do more mature mothers and those who gave birth via normal vaginal delivery. The study also concluded that the lower the birth order the less the delay and singleton babies were delayed less by their parents. Finally the current study recommended the activation of central guidelines that provide clear directions to help improve immunization coverage of such high risk infants in addition to increased efforts by all sectors to increase public awareness about the importance of compliance with up-to-date immunizations. .