Objective Haemodialysis patients are at risk of serious health complications, yet treatment non-adherence remains high. Warnings about health-risks associated with non-adherence may trigger defensive reactions. We studied whether an intervention based on self-affirmation theory reduced resistance to health-risk information and improved fluid treatment adherence. Design In a cluster randomised controlled trial, ninety-one patients either self-affirmed or completed a matched-control task before reading about the health-risks associated with inadequate fluid control. Outcome measures Patients' perceptions of the health-risk information, intention and self-efficacy to control fluid, were assessed immediately after presentation of health-risk information. Interdialytic-weight-gain (IDWG), excess fluid removed during hemodialysis, is a clinical measure of fluid treatment adherence. IDWG data were collected up to 12 months post intervention. Results Self-affirmed patients had significantly reduced IDWG levels over 12 months. However, contrary to predictions derived from self-affirmation theory, self-affirmed participants and controls did not differ in their evaluation of the health-risk information, intention to control fluid or self-efficacy. Conclusion A low-cost, high-reach health intervention based on self-affirmation theory was shown to reduce IDWG over a 12-month period but the mechanism by which this apparent behaviour change occurred is uncertain. Further work is still required to identify mediators of the observed effects.