Aim: In Denmark, preterm infants are recommended to receive childhood vaccinations without correction for gestational age. This study aimed to describe the timeliness of the Danish Childhood Vaccination Program in preterm infants during the first 13 months of life and to evaluate possible determinants of delay. Methods: This retrospective cohort study included preterm infants admitted to a level III neonatal intensive care unit between October 2019 and October 2020. Clinical data were retrieved from medical records and the Danish Vaccination Register. Timely vaccination was defined corresponding to chronological age of 3-, 5- and 12 months, within a time interval of 30 days before to 29 days after the expected date. Results: Analyses included 365 infants. Timely vaccination occurred in 91%, 83% and 67% of preterm infants for the first, second and third vaccination, respectively, and timeliness was highest if born before gestational age 28 weeks. Gestational age 28–31 + 6 weeks and delayed former vaccinations negatively influenced the timeliness of the following vaccinations. Conclusion: Most preterm infants received the first vaccination timely; however, timeliness decreased with each subsequent vaccination. Efforts to improve timeliness should focus on counselling healthcare personnel and parents to follow the recommendations for the first and the following vaccinations.