IntroductionWomen with premenstrual syndrome (PMS) show majority of symptoms, particularly during luteal phase, regularly every month. These symptoms are classified into 3 emotional (EM), physical (PHY) and behavioral (BE) categories (1). The common symptoms of PMS are: bloating, breast tenderness, headache, frequent urination, loss of appetite, insomnia, irritability, depression, lack of energy and increased or decreased of libido (1,2). Andrew et al (3) have shown that patients with PMS have lower work performance (27.5%), impaired working relationships (23.1%), disputation with their husbands (8.82%), relationship problems with their children (6%) and social relationship problems (41%). PMS interferes with the life of patients directly by treatment expenditures and indirectly by inefficient working hours, absenteeism and productivity (4) and requires cure and management (5). Routinely, women with PMS use pharmacological (1) and non-pharmacological approaches (6) to decrease the severity of PMS symptoms. Whereas, prescribed medications (alprazolam, progesterone, fluoxetine and gonadotropin releasing hormone) have shown side effects such as exhaustion, insomnia, headaches, menstrual dysfunction, decreased sexual ability, nausea and vomiting (1,(7)(8)(9). Recently, yoga as a safe treatment has also been introduced (10).Yoga is a Sanskrit word, meaning connection of the mind, body, emotions, logic and attention to the action. Yoga exercise is composed of physical (Asana), breathing (Pranayama) and mental (Pratt Hara) practices, resulting in health, relaxation and positive awareness. Yoga stretching practices with deep breathing and meditation strengthens muscles and protects the spine and joints of body (11). Evidences indicated that physical and mental health benefits of yoga occur through negative moderation of hypothalamic-pituitary-adrenal axis in association with sympathetic nervous system (SNS). Yoga exercise, negatively, via vagal nerves, regulates secretion of hormones (cortisol, glucose, plasma ҆ s renin, epinephrine and norepinephrine) into the bloodstream which are responsible for body functions (10). Yoga reduces the negative effects of induced stress to immune system by regulating positively the adjustment of immunoglobulin A (12). Additionally, yoga exercise reduces harmful inflammatory secretions, which make women with PMS, comfortable. Studies in India have shown positive impacts of yoga exercise on women with PMS on decreasing heart beats, blood pressure and anxiety (6,13). AbstractObjectives: Women with premenstrual syndrome (PMS) show emotional, physical and behavioral symptoms regularly every month. Mild physiological symptoms of PMS appear in 95% of women at their reproductive age. We hypothesized to evaluate the effect of yoga exercise on PMS. Materials and Methods: A randomized controlled clinical trial was conducted from April to October of 2015. Eligible women for inclusion were selected conveniently from women that had randomly referred to selected private obstetrics and gynecology clini...