Objective We aimed to assess informed decision making about childhood immunization by measuring knowledge, deliberation and value-consistency. Additionally, we investigated whether informed decision making is different for parents who accept and parents who decline vaccination. Methods Parents of at least one child between 3 months and 3,5 years of age were randomly selected from a vaccination register and were asked to fill in an online questionnaire measuring informed decision making. Results In total, 1615 parents completed the questionnaire (16.2%). 77.6% of the parents were classified as having sufficient knowledge, 34.3% of the decisions were deliberate, and 94% were value-consistent. This resulted in 21% of parents who were classified as making an informed decision. Vaccination decliners more often made a decision classified as informed than vaccination acceptors (34.3% vs. 19.7%). When decliners made a decision classified as uninformed, this was mainly due to insufficient knowledge, while uninformed decisions among acceptors were mainly due to a low level of deliberation. Conclusion and practice implications Most parents made an uninformed decision about childhood vaccinations. Decision aids and counseling by child vaccine providers could improve the knowledge of decliners and motivate acceptors to deliberate about their decision, and by that promote informed decision making.