This study tested the hypothesis that commonly reported negative effects of sugar on children's behavior may be due to parental expectancies. A challenge study design was employed, in which thirty-five 5- to 7-year-old boys reported by their mothers to be behaviorally "sugar sensitive," and their mothers, were randomly assigned to experimental and control groups. In the experimental group, mothers were told their children had received a large dose of sugar, whereas in the control condition mothers were told their sons received a placebo; all children actually received the placebo (aspartame). Mothers and sons were videotaped while interacting together and each mother was then questioned about the interaction. Mothers in the sugar expectancy condition rated their children as significantly more hyperactive. Behavioral observations revealed these mothers exercised more control by maintaining physical closeness, as well as showing trends to criticize, look at, and talk to their sons more than did control mothers. For several variables, the expectancy effect was stronger for cognitively rigid mothers.