This research examined whether mothers' and fathers' beliefs about their children's alcohol use had cumulative self-fulfilling effects on their children's future drinking behavior. Analyses of longitudinal data acquired from 115 seventh-grade children and their mothers and fathers were consistent with synergistic accumulation effects for negative beliefs: Parents' beliefs predicted the greatest degree of confirmatory behavior from children when both mothers and fathers overestimated their children's alcohol use. Results did not support synergistic accumulation effects for positive beliefs: Children's predicted future alcohol use was similar regardless of whether one parent or both underestimated their child's alcohol use. These findings suggest that the generally small self-fulfilling effects reported in the literature may underestimate the power of negative self-fulfilling prophecies to harm targets because studies have not taken into consideration the possibility that negative self-fulfilling prophecies may be more likely than positive ones to accumulate across multiple perceivers.