Most current efforts to enhance critical thinking focus on skills practice and training. The empirical research from the fields of cognition and affect sciences suggest that positive mood, even when transiently induced, can have beneficial effects on cognitive flexibility and problem solving. We undertook this study to test this hypothesis in a practical setting. Using an A-B-A-B within subject design, we measured the impact of positive (versus neutral) mood on critical thinking demonstrated on four essay exams in an undergraduate course in personality. There was a significant enhancing effect of positive mood on critical thinking in female students, but not in male students. We discuss possible sex differences that may account for the partial support of the mood-critical thinking effect.