Genetic testing for obesity risk is increasingly available to the public but few studies have examined motivational or affective reactions. Here we report findings from a "vignette" study investigating reactions to "higher-risk" and "average-risk" results for the obesity-related FTO gene in two groups: a panel sample of individuals with weight concerns, for whom testing may have treatment implications (n = 306, mean age = 45 years, mean BMI = 35) and a student sample (n = 395, mean age = 25 years, mean BMI = 23), for whom testing would have implications for obesity prevention. Participants were given FTO gene information that described higher-risk alleles as linked with modest weight gain and slightly higher risk of obesity. They responded to both higher- and average-risk vignettes, with order randomized. Interest in genetic testing was high overall, and higher in panel respondents than students (93% vs. 78% would "probably" or "definitely" have the test; P < 0.001). In students, a higher-risk result generated higher motivation to change (d = 0.15; P < 0.001), but also slightly higher negative affect (d = 0.03, P < 0.001) and fatalism (d = 0.05, P < 0.001) than an average-risk result. Panel respondents also had higher motivation to change (d = 0.17, P < 0.001) as well as relief about having an explanation for their body weight (d = 0.02, P = 0.013) in the higher-risk condition, but no increase in fatalism or depression. These results suggest that at the level of anticipated responses to FTO gene feedback, higher-risk results had positive motivational effects with minimal changes in negative affect or fatalism. Genetic testing has the potential to be a useful clinical or preventive tool when combined with appropriate information.