Premenstrual Syndrome (PMS) has been a subject of great interest in the last few years and has attracted many theories and treatments. Most of these focus on physical disturbance or imbalance. Less work has been done on psychological mechanisms associated with PMS.A cognitive model is described that considers a broad range of factors that contribute to those premenstrual affective changes associated with Late Luteal Phase Dysphoric Disorder (LLPDD) now called Premenstrual Dysphoric Disorder (PMDD). It is proposed that the cognitive appraisal of premenstrual changes in the context of a woman's current circumstances and personal assumptions determines whether she perceives them as distressing or as a normal and manageable part of her life. Vicious circles of negative thinking magnify the symptoms and increase anxiety, irritability, and low mood.Cognitive therapy gives a sufferer of PMS the opportunity to review her responses and to experiment with more adaptive thoughts and behavior. The process of therapy is described. This method was developed in the course of a treatment study for women with PMS. The treated group had significant relief of premenstrual symptoms.