To identify the determinants of influenza vaccination and the moderators of the intention-behavior relationship among healthcare workers (HCWs). Prospective survey with 2-month follow-up. Three university-affiliated public hospitals. Random sample of 424 HCWs. The intention of an HCW to get vaccinated against influenza was measured by means of a self-administered questionnaire based on an extended version of the theory of planned behavior. An objective measure of behavior was extracted 2 months later from the vaccination database of the hospitals. Controlling for past behavior, we found that the determinants of influenza vaccination were intention (odds ratio [OR], 8.32 [95% confidence interval {CI}, 2.82-24.50]), moral norm (OR, 3.01 [95% CI, 1.17-7.76]), anticipated regret (OR, 2.33 [95% CI, 1.23-4.41]), and work status (ie, full time vs part time; OR, 1.99 [95% CI, 1.92-3.29]). Moral norm also interacted with intention as a significant moderator of the intention-behavior relationship (OR, 0.09 [95% CI, 0.03-0.30]). Again, apart from the influence of past behavior, intention to get vaccinated was predicted by use of the following variables: attitude (beta=.32; P<.001), professional norm (beta=.18; P<.001), moral norm (beta=.18; P<.001), subjective norm (beta=.09; P<.001), and self-efficacy (beta=.08; P<.001). This latter model explained 89% of the variance in HCWs' intentions to get vaccinated against influenza during the next vaccination campaign. Our study suggests that influenza vaccination among HCWs is mainly a motivational issue. In this regard, it can be suggested to reinforce the idea that getting vaccinated can reduce worry and protect family members.