Anti-fat sentiment is increasing, is prevalent in health professionals, and has health and social consequences. There is no evidence for effective obesity prejudice reduction techniques in health professionals. The present experiment sought to reduce implicit and explicit anti-fat prejudice in preservice health students. Health promotion/public health bachelor degree program students (n = 159) were randomized to one of three tutorial conditions. One condition presented an obesity curriculum on the controllable reasons for obesity (i.e., diet/exercise). A prejudice reduction condition presented evidence on the uncontrollable reasons for obesity (i.e., genes/environment); whereas a neutral (control) curriculum focused on alcohol use in young people. Measures of implicit and explicit anti-fat prejudice, beliefs about obese people, and dieting, were taken at baseline and postintervention. Repeated measures analyses showed decreases in two forms of implicit anti-fat prejudice (decreases of 27 and 12%) in the genes/environment condition relative to other conditions. The diet/exercise condition showed a 27% increase in one measure of implicit anti-fat prejudice. Reductions in explicit anti-fat prejudice were also seen in the genes/environment condition (P = 0.006). No significant changes in beliefs about obese people or dieting control beliefs were found across conditions. The present results show that anti-fat prejudice can be reduced or exacerbated depending on the causal information provided about obesity. The present results have implications for the training of health professionals, especially given their widespread negativity toward overweight and obesity.