Rates of flu vaccination among healthcare professionals often remain lower than recommended guidelines. We tested whether autonomy-supportive communication styles could improve the effectiveness of statements seeking to promote professionals’ flu vaccination uptake. A pilot study established that statements presented in an autonomy-supportive communication style (i.e., upholding freedom of choice) posed a significantly lower threat to freedom compared to equivalent statements presented in a controlling communication style (i.e., thwarting choice by implying obligation). The main experiment examined the impact of these two communication styles on healthcare professionals’ behavioural intentions to vaccinate against the flu. Results replicated the dampening effect of autonomy-supportive communication style on perceived threat to freedom. Crucially, only autonomy-supportive communication styles led to a significant increase of behavioural intentions to vaccinate. Furthermore, this effect was moderated by motivational regulations (measured by the TSRQ-flu scale): it was strongest for those who tended to see flu vaccination as unimportant and unconnected with their internal values (low autonomous regulation), those who tended not to see vaccination as an act that would give them pride or reduce guilt (low introjection) or who tended to be unwilling to act to get vaccinated (high amotivation). Implications for future policy or institution-led communication campaigns are discussed.