Violence and aggressiveness are social concerns. Also, at a time of rising prevalence of obesity, many people tend to control their body weight through dieting. We analyzed the impact of weight loss on aggressiveness: 150 participants completed anonymously two questionnaires assessing their aggressiveness, age, sex, diet, recent body weight change, reasons of recent body weight changes, and perceived difficulties related to those changes. Results showed that participants who had deliberately lost weight reported higher aggressiveness than controls, but passive weight-losers did not. The raised aggressiveness was stronger for hostile aggression than for instrumental aggression. Such a rise is likely to be due to the discomfort associated with opposing body weight set-point.