In two experiments, 4- to 9-year-olds played a game in which they selected one of two boxes to win a prize. On "regret" trials the unchosen box contained a better prize than the prize children actually won, and on "baseline" trials the other box contained a prize of the same value. Children rated their feelings about their prize before and after seeing what they could have won if they had chosen the other box and were asked to provide an explanation if their feelings had changed. Patterns of responding suggested that regret was experienced by 6 or 7 years of age; children of this age could also explain why they felt worse in regret trials by referring to the counterfactual situation in which the prize was better. No evidence of regret was found in 4- and 5-year-olds. Additional findings suggested that by 6 or 7 years, children's emotions were determined by a consideration of two different counterfactual scenarios. (Contains 1 table and 1 figure.)