ABSTRACT Purpose: Employee sickness absence increased steadily in recent years posing an important challenge for organizations and research. The present study addresses this need by analysing the causal link of transformational leadership and sickness absence, while also taking into consideration the context of leadership by studying the moderation effect of a leader's perceived organizational support (POS) in that association. Design/Method: 57 middle managers of two Spanish companies were randomly assigned to either the control (n = 22) or intervention (n = 35) condition. Multigroup linear regression in MPlus was carried out to compare the effect of transformational leadership on follower sick leave during a 6 months pre and post-test period for both groups, while also taking into account the moderation effect of a leader's POS. Findings: The analysis confirmed the association between transformational leadership and sick leave days due to short-term spells, moderated by the leader's levels of POS. Moreover, this relationship was stronger within the intervention as compared to the control group. Conclusion: The present research provides causal evidence for the link between transformational leadership and sickness absence, and offers an evidence-based and actionable leadership training method for organizations seeking to reduce employee sick leave.