Objective: Positive mood on the day of vaccination has been associated with subsequent antibody responses to the influenza vaccine in older adults. The primary aim of this trial was to examine whether a brief intervention was able to enhance positive mood at the time of vaccination in a clinical context. Secondary aims included exploratory analyses of the effects of the intervention on nonspecific and influenza-specific immunity. Method: One hundred three older adults (65-85 years) participated in a 2-arm, parallel, single-blind, randomized controlled trial. Participants viewed either a 15-min video package designed to induce positive mood or a matched neutral control video, immediately prior to receiving a standard dose quadrivalent influenza vaccination. State affect and secretory immunoglobulin A levels were assessed immediately prior to, and following, the interventions. Antigen-specific immunoglobulin G responses to the vaccination were assessed at 4 and 16 weeks postvaccination. Results: The positive mood intervention resulted in significant improvements in state positive affect, compared with the neutral control. Secretory immunoglobulin A levels significantly increased across both groups. Antigen-specific immunoglobulin G responses to influenza vaccination were not statistically significantly different between groups, although point estimates of effect size favored participants who viewed the positive mood intervention for most strains at both 4 and 16 weeks postvaccination. Conclusions: A 15-min intervention can improve positive mood in older adults prior to vaccination. Future trials should examine whether enhancing mood at the time of vaccination could enhance the effectiveness of influenza vaccination on patients and benefit health services.