The role of expectancy in enhancing or mitigating premenstrual symptoms was examined in 51 mothers of preschoolers. Expectancy was manipulated by providing information either in support of a biological cause for genuine universal mood changes or in support of a psychological cause arising out of negative societal myths. A third group was given no information. Mood, cognitive function, and mother-child interaction were assessed both at mid-cycle and premenstrually, and the results suggested that expectancy enhances symptoms. The Psychological group lowered their symptom expectations and reported less negative mood premenstrually as well as fewer symptoms at the end of the test month. The other groups reported no change in symptoms and greater premenstrual negative mood than the Psychological group. All groups demonstrated greater task persistence and more positive mother-child interaction during the premenstrual phase.