Studies have shown that parents reduce control and support in response to youths' drinking. Why they react this way, however, is still unknown. From cognitive dissonance theory, we derived hypotheses about parents' reactions. We used a longitudinal, school-based sample of 494 youths (13 and 14 years, 56% boys) and their parents. General Linear Model (GLM) analyses were used to test the main hypotheses. In accord with our hypotheses, parents who encountered their youths intoxicated became less opposed to underage drinking over time. In addition, parents who remained strongly opposed to youth drinking experienced more worries than parents who became less opposed. Alternative explanations for the results were tested, but were not supported. The findings suggest that to eliminate the dissonance between their strict attitudes against youth drinking and their knowledge of their own youths' drinking, parents changed their attitudes and became more lenient. (Contains 2 tables and 3 figures.)