Uptake of MMR vaccinations is as low as 60% in some parts of the UK. This poses a serious public health issue. This longitudinal study investigates parental decisions about MMR and single vaccinations. Parents (n = 114) rated their perceptions of the benefits and risks of immunisation, and emotion-related variables; and were followed up to ask their final immunisation decision. Analyses demonstrated that parental decisions were explained by emotion-related variables, specifically anticipated responsibility and regret. It was concluded that parents’ decisions about MMR are strongly influenced by the idea than harm that occurs as a result of deciding to immunise (commission) is less acceptable than harm that occurs as a result of deciding not to immunise (omission) (known as ‘omission bias’).